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Color

Color (American English) or colour (British and Commonwealth English) is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorption, reflection, emission spectra, and interference. For most humans, colors are perceived in the visible light spectrum with three types of cone cells (trichromacy). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelengths, such as bees that can distinguish ultraviolet, and thus have a different color sensitivity range. Animal perception of color originates from different light wavelength or spectral sensitivity in cone cell types, which is then processed by the brain.

Colors have perceived properties such as hue, colorfulness (saturation), and luminance. Colors can also be additively mixed (commonly used for actual light) or subtractively mixed (commonly used for materials). If the colors are mixed in the right proportions, because of metamerism, they may look the same as a single-wavelength light. For convenience, colors can be organized in a color space, which when being abstracted as a mathematical color model can assign each region of color with a corresponding set of numbers. As such, color spaces are an essential tool for color reproduction in print, photography, computer monitors, and television. The most well-known color models are RGB, CMYK, YUV, HSL, and HSV.

Because the perception of color is an important aspect of human life, different colors have been associated with emotions, activity, and nationality. Names of color regions in different cultures can have different, sometimes overlapping areas. In visual arts, color theory is used to govern the use of colors in an aesthetically pleasing and harmonious way. The theory of color includes the color complements; color balance; and classification of primary colors (traditionally red, yellow, blue), secondary colors (traditionally orange, green, purple), and tertiary colors. The study of colors in general is called color science.

Physical properties
Electromagnetic radiation is characterized by its wavelength (or frequency) and its intensity. When the wavelength is within the visible spectrum (the range of wavelengths humans can perceive, approximately from 390 nm to 700 nm), it is known as "visible light".

Most light sources emit light at many different wavelengths; a source's spectrum is a distribution giving its intensity at each wavelength. Although the spectrum of light arriving at the eye from a given direction determines the color sensation in that direction, there are many more possible spectral combinations than color sensations. In fact, one may formally define a color as a class of spectra that give rise to the same color sensation, although such classes would vary widely among different species, and to a lesser extent among individuals within the same species. In each such class, the members are called metamers of the color in question. This effect can be visualized by comparing the light sources' spectral power distributions and the resulting colors.

Spectral colors
The familiar colors of the rainbow in the spectrum—named using the Latin word for appearance or apparition by Isaac Newton in 1671—include all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The spectrum above shows approximate wavelengths (in nm) for spectral colors in the visible range. Spectral colors have 100% purity, and are fully saturated. A complex mixture of spectral colors can be used to describe any color, which is the definition of a light power spectrum.

The spectral colors form a continuous spectrum, and how it is divided into distinct colors linguistically is a matter of culture and historical contingency. Despite the ubiquitous ROYGBIV mnemonic used to remember the spectral colors in English, the inclusion or exclusion of colors is contentious, with disagreement often focused on indigo and cyan. Even if the subset of color terms is agreed, their wavelength ranges and borders between them may not be.

The intensity of a spectral color, relative to the context in which it is viewed, may alter its perception considerably. For example a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive green. Additionally, hue shifts towards yellow or blue happen if the intensity of a spectral light is increased; this is called Bezold–Brücke shift. In color models capable of representing spectral colors, such as CIELUV, a spectral color has the maximal saturation. In Helmholtz coordinates, this is described as 100% purity.

Color of objects
The physical color of an object depends on how it absorbs and scatters light. Most objects scatter light to some degree and do not reflect or transmit light specularly like glasses or mirrors. A transparent object allows almost all light to transmit or pass through, thus transparent objects are perceived as colorless. Conversely, an opaque object does not allow light to transmit through and instead absorbing or reflecting the light it receives. Like transparent objects, translucent objects allow light to transmit through, but translucent objects are seen colored because they scatter or absorb certain wavelengths of light via internal scatterance. The absorbed light is often dissipated as heat.

Development of theories of color vision
Although Aristotle and other ancient scientists had already written on the nature of light and color vision, it was not until Newton that light was identified as the source of the color sensation. In 1810, Goethe published his comprehensive Theory of Colors in which he provided a rational description of color experience, which 'tells us how it originates, not what it is'. (Schopenhauer)

In 1801 Thomas Young proposed his trichromatic theory, based on the observation that any color could be matched with a combination of three lights. This theory was later refined by James Clerk Maxwell and Hermann von Helmholtz. As Helmholtz puts it, "the principles of Newton's law of mixture were experimentally confirmed by Maxwell in 1856. Young's theory of color sensations, like so much else that this marvelous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it."

At the same time as Helmholtz, Ewald Hering developed the opponent process theory of color, noting that color blindness and afterimages typically come in opponent pairs (red-green, blue-orange, yellow-violet, and black-white). Ultimately these two theories were synthesized in 1957 by Hurvich and Jameson, who showed that retinal processing corresponds to the trichromatic theory, while processing at the level of the lateral geniculate nucleus corresponds to the opponent theory.

In 1931, an international group of experts known as the Commission internationale de l'éclairage (CIE) developed a mathematical color model, which mapped out the space of observable colors and assigned a set of three numbers to each.

Color in the eye

The ability of the human eye to distinguish colors is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans are trichromatic—the retina contains three types of color receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that is perceived as blue or blue-violet, with wavelengths around 450 nm; cones of this type are sometimes called short-wavelength cones or S cones (or misleadingly, blue cones). The other two types are closely related genetically and chemically: middle-wavelength cones, M cones, or green cones are most sensitive to light perceived as green, with wavelengths around 540 nm, while the long-wavelength cones, L cones, or red cones, are most sensitive to light that is perceived as greenish yellow, with wavelengths around 570 nm.

Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. Each cone type adheres to the principle of univariance, which is that each cone's output is determined by the amount of light that falls on it over all wavelengths. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These amounts of stimulation are sometimes called tristimulus values.

The response curve as a function of wavelength varies for each type of cone. Because the curves overlap, some tristimulus values do not occur for any incoming light combination. For example, it is not possible to stimulate only the mid-wavelength (so-called "green") cones; the other cones will inevitably be stimulated to some degree at the same time. The set of all possible tristimulus values determines the human color space. It has been estimated that humans can distinguish roughly 10 million different colors.

The other type of light-sensitive cell in the eye, the rod, has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all. On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a colorless response (furthermore, the rods are barely sensitive to light in the "red" range). In certain conditions of intermediate illumination, the rod response and a weak cone response can together result in color discriminations not accounted for by cone responses alone. These effects, combined, are summarized also in the Kruithof curve, which describes the change of color perception and pleasingness of light as a function of temperature and intensity.

Color in the brain
While the mechanisms of color vision at the level of the retina are well-described in terms of tristimulus values, color processing after that point is organized differently. A dominant theory of color vision proposes that color information is transmitted out of the eye by three opponent processes, or opponent channels, each constructed from the raw output of the cones: a red–green channel, a blue–yellow channel, and a black–white "luminance" channel. This theory has been supported by neurobiology, and accounts for the structure of our subjective color experience. Specifically, it explains why humans cannot perceive a "reddish green" or "yellowish blue", and it predicts the color wheel: it is the collection of colors for which at least one of the two color channels measures a value at one of its extremes.

The exact nature of color perception beyond the processing already described, and indeed the status of color as a feature of the perceived world or rather as a feature of our perception of the world—a type of qualia—is a matter of complex and continuing philosophical dispute. From the V1 blobs, color information is sent to cells in the second visual area, V2. The cells in V2 that are most strongly color tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem to be concerned with other visual information like motion and high-resolution form). Neurons in V2 then synapse onto cells in the extended V4. This area includes not only V4, but two other areas in the posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex, and posterior TEO. Area V4 was initially suggested by Semir Zeki to be exclusively dedicated to color, and he later showed that V4 can be subdivided into subregions with very high concentrations of color cells separated from each other by zones with lower concentration of such cells though even the latter cells respond better to some wavelengths than to others, a finding confirmed by subsequent studies. The presence in V4 of orientation-selective cells led to the view that V4 is involved in processing both color and form associated with color but it is worth noting that the orientation selective cells within V4 are more broadly tuned than their counterparts in V1, V2, and V3. Color processing in the extended V4 occurs in millimeter-sized color modules called globs. This is the part of the brain in which color is first processed into the full range of hues found in color space.

Color vision deficiency
A color vision deficiency causes an individual to perceive a smaller gamut of colors than the standard observer with normal color vision. The effect can be mild, having lower "color resolution" (i.e. anomalous trichromacy), moderate, lacking an entire dimension or channel of color (e.g. dichromacy), or complete, lacking all color perception (i.e. monochromacy). Most forms of color blindness derive from one or more of the three classes of cone cells either being missing, having a shifted spectral sensitivity or having lower responsiveness to incoming light. In addition, cerebral achromatopsia is caused by neural anomalies in those parts of the brain where visual processing takes place.

Some colors that appear distinct to an individual with normal color vision will appear metameric to the color blind. The most common form of color blindness is congenital red–green color blindness, affecting ~8% of males. Individuals with the strongest form of this condition (dichromacy) will experience blue and purple, green and yellow, teal, and gray as colors of confusion, i.e. metamers.

Tetrachromacy
Outside of humans, which are mostly trichromatic (having three types of cones), most mammals are dichromatic, possessing only two cones. However, outside of mammals, most vertebrate are tetrachromatic, having four types of cones, and includes most, birds,  reptiles, amphibians, and bony fish. An extra dimension of color vision means these vertebrates can see two distinct colors that a normal human would view as metamers. Some invertebrates, such as the mantis shrimp, have an even higher number of cones (12) that could lead to a richer color gamut than even imaginable by humans.

The existence of human tetrachromats is a contentious notion. As many as half of all human females have 4 distinct cone classes, which could enable tetrachromacy. However, a distinction must be made between retinal (or weak) tetrachromats, which express four cone classes in the retina, and functional (or strong) tetrachromats, which are able to make the enhanced color discriminations expected of tetrachromats. In fact, there is only one peer-reviewed report of a functional tetrachromat. It is estimated that while the average person is able to see one million colors, someone with functional tetrachromacy could see a hundred million colors.

Synesthesia
In certain forms of synesthesia, perceiving letters and numbers (grapheme–color synesthesia) or hearing sounds (chromesthesia) will evoke a perception of color. Behavioral and functional neuroimaging experiments have demonstrated that these color experiences lead to changes in behavioral tasks and lead to increased activation of brain regions involved in color perception, thus demonstrating their reality, and similarity to real color percepts, albeit evoked through a non-standard route. Synesthesia can occur genetically, with 4% of the population having variants associated with the condition. Synesthesia has also been known to occur with brain damage, drugs, and sensory deprivation.

The philosopher Pythagoras experienced synesthesia and provided one of the first written accounts of the condition in approximately 550 BCE. He created mathematical equations for musical notes that could form part of a scale, such as an octave.

Afterimages
After exposure to strong light in their sensitivity range, photoreceptors of a given type become desensitized. For a few seconds after the light ceases, they will continue to signal less strongly than they otherwise would. Colors observed during that period will appear to lack the color component detected by the desensitized photoreceptors. This effect is responsible for the phenomenon of afterimages, in which the eye may continue to see a bright figure after looking away from it, but in a complementary color. Afterimage effects have also been used by artists, including Vincent van Gogh.

Color constancy
When an artist uses a limited color palette, the human visual system tends to compensate by seeing any gray or neutral color as the color which is missing from the color wheel. For example, in a limited palette consisting of red, yellow, black, and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure gray will appear bluish.

The trichromatic theory is strictly true when the visual system is in a fixed state of adaptation. In reality, the visual system is constantly adapting to changes in the environment and compares the various colors in a scene to reduce the effects of the illumination. If a scene is illuminated with one light, and then with another, as long as the difference between the light sources stays within a reasonable range, the colors in the scene appear relatively constant to us. This was studied by Edwin H. Land in the 1970s and led to his retinex theory of color constancy.

Both phenomena are readily explained and mathematically modeled with modern theories of chromatic adaptation and color appearance (e.g. CIECAM02, iCAM). There is no need to dismiss the trichromatic theory of vision, but rather it can be enhanced with an understanding of how the visual system adapts to changes in the viewing environment.

Reproduction

Color reproduction is the science of creating colors for the human eye that faithfully represent the desired color. It focuses on how to construct a spectrum of wavelengths that will best evoke a certain color in an observer. Most colors are not spectral colors, meaning they are mixtures of various wavelengths of light. However, these non-spectral colors are often described by their dominant wavelength, which identifies the single wavelength of light that produces a sensation most similar to the non-spectral color. Dominant wavelength is roughly akin to hue.

There are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (mixtures of red and violet light, from opposite ends of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray, and white) and colors such as pink, tan, and magenta.

Two different light spectra that have the same effect on the three color receptors in the human eye will be perceived as the same color. They are metamers of that color. This is exemplified by the white light emitted by fluorescent lamps, which typically has a spectrum of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although the color rendering index of each light source may affect the color of objects illuminated by these metameric light sources.

Similarly, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photography, printing, television, and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colors. Each method has its advantages and disadvantages depending on the particular application.

No mixture of colors, however, can produce a response truly identical to that of a spectral color, although one can get close, especially for the longer wavelengths, where the CIE 1931 color space chromaticity diagram has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.

Because of this, and because the primaries in color printing systems generally are not pure themselves, the colors reproduced are never perfectly saturated spectral colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut.

Another problem with color reproduction systems is connected with the initial measurement of color, or colorimetry. The characteristics of the color sensors in measurement devices (e.g. cameras, scanners) are often very far from the characteristics of the receptors in the human eye.

A color reproduction system "tuned" to a human with normal color vision may give very inaccurate results for other observers, according to color vision deviations to the standard observer.

The different color response of different devices can be problematic if not properly managed. For color information stored and transferred in digital form, color management techniques, such as those based on ICC profiles, can help to avoid distortions of the reproduced colors. Color management does not circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colors into the gamut that can be reproduced.

Additive coloring
Additive color is light created by mixing together light of two or more different colors. Red, green, and blue are the additive primary colors normally used in additive color systems such as projectors, televisions, and computer terminals.

Subtractive coloring
Subtractive coloring uses dyes, inks, pigments, or filters to absorb some wavelengths of light and not others. The color that a surface displays comes from the parts of the visible spectrum that are not absorbed and therefore remain visible. Without pigments or dye, fabric fibers, paint base and paper are usually made of particles that scatter white light (all colors) well in all directions. When a pigment or ink is added, wavelengths are absorbed or "subtracted" from white light, so light of another color reaches the eye.

If the light is not a pure white source (the case of nearly all forms of artificial lighting), the resulting spectrum will appear a slightly different color. Red paint, viewed under blue light, may appear black. Red paint is red because it scatters only the red components of the spectrum. If red paint is illuminated by blue light, it will be absorbed by the red paint, creating the appearance of a black object.

The subtractive model also predicts the color resulting from a mixture of paints, or similar medium such as fabric dye, whether applied in layers or mixed together prior to application. In the case of paint mixed before application, incident light interacts with many different pigment particles at various depths inside the paint layer before emerging.

Structural color
Structural colors are colors caused by interference effects rather than by pigments. Color effects are produced when a material is scored with fine parallel lines, formed of one or more parallel thin layers, or otherwise composed of microstructures on the scale of the color's wavelength. If the microstructures are spaced randomly, light of shorter wavelengths will be scattered preferentially to produce Tyndall effect colors: the blue of the sky (Rayleigh scattering, caused by structures much smaller than the wavelength of light, in this case, air molecules), the luster of opals, and the blue of human irises. If the microstructures are aligned in arrays, for example, the array of pits in a CD, they behave as a diffraction grating: the grating reflects different wavelengths in different directions due to interference phenomena, separating mixed "white" light into light of different wavelengths. If the structure is one or more thin layers then it will reflect some wavelengths and transmit others, depending on the layers' thickness.

Structural color is studied in the field of thin-film optics. The most ordered or the most changeable structural colors are iridescent. Structural color is responsible for the blues and greens of the feathers of many birds (the blue jay, for example), as well as certain butterfly wings and beetle shells. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in peacock feathers, soap bubbles, films of oil, and mother of pearl, because the reflected color depends upon the viewing angle. Numerous scientists have carried out research in butterfly wings and beetle shells, including Isaac Newton and Robert Hooke. Since 1942, electron micrography has been used, advancing the development of products that exploit structural color, such as "photonic" cosmetics.

Optimal colors
The gamut of the human color vision is bounded by optimal colors. They are the most chromatic colors that humans are able to see.

The emission or reflectance spectrum of a color is the amount of light of each wavelength that it emits or reflects, in proportion to a given maximum, which has the value of 1 (100%). If the emission or reflectance spectrum of a color is 0 (0%) or 1 (100%) across the entire visible spectrum, and it has no more than two transitions between 0 and 1, or 1 and 0, then it is an optimal color. With the current state of technology, we are unable to produce any material or pigment with these properties.

Thus, four types of "optimal color" spectra are possible: In the first, the transition goes from 0 at both ends of the spectrum to 1 in the middle, as shown in the image at right. In the second, it goes from 1 at the ends to 0 in the middle. In the third type, it starts at 1 at the red end of the spectrum, and it changes to 0 at a given wavelength. In the fourth type, it starts at 0 in the red end of the spectrum, and it changes to 1 at a given wavelength. The first type produces colors that are similar to the spectral colors and follow roughly the horseshoe-shaped portion of the CIE xy chromaticity diagram (the spectral locus), but are generally more chromatic, although less spectrally pure. The second type produces colors that are similar to (but generally more chromatic and less spectrally pure than) the colors on the straight line in the CIE xy chromaticity diagram (the "line of purples"), leading to magenta or purple-like colors. The third type produces the colors located in the "warm" sharp edge of the optimal color solid (this will be explained later in the article). The fourth type produces the colors located in the "cold" sharp edge of the optimal color solid.

The optimal color solid, Rösch–MacAdam color solid, or simply visible gamut, is a type of color solid that contains all the colors that humans are able to see. The optimal color solid is bounded by the set of all optimal colors.

In optimal color solids, the colors of the visible spectrum are theoretically black, because their emission or reflection spectrum is 1 (100%) in only one wavelength, and 0 in all of the other infinite visible wavelengths that there are, meaning that they have a lightness of 0 with respect to white, and will also have 0 chroma, but, of course, 100% of spectral purity. In short: In optimal color solids, spectral colors are equivalent to black (0% lightness, 0 chroma), but have full spectral purity (they are located in the horseshoe-shaped spectral locus of the chromaticiy diagram).

In linear color spaces that contain all colors visible by humans, such as LMS or CIE 1931 XYZ, the set of half-lines that start at the origin (black, (0, 0, 0)) and pass through all the points that represent the colors of the visible spectrum, and the portion of a plane that passes through the violet half-line and the red half-line (both ends of the visible spectrum), generate the "spectrum cone". The black point (coordinates (0, 0, 0)) of the optimal color solid (and only the black point) is tangent to the "spectrum cone", and the white point (1, 1, 1) (only the white point) is tangent to the "inverted spectrum cone", with the "inverted spectrum cone" being symmetrical to the "spectrum cone" with respect to the middle gray point (0.5, 0.5, 0.5). This means that, in linear color spaces, the optimal color solid is centrally symmetric.

In most color spaces, the surface of the optimal color solid is smooth, except for two points (black and white); and two sharp edges: the "warm" edge, which goes from black, to red, to orange, to yellow, to white; and the "cold" edge, which goes from black, to deep violet, to blue, to cyan, to white. This is due to the following: If the portion of the emission or reflection spectrum of a color is spectral red (which is located at one end of the spectrum), it will be seen as black. If the size of the portion of total emission or reflectance is increased, now covering from the red end of the spectrum to the yellow wavelengths, it will be seen as red or orange. If the portion is expanded more, covering the green wavelengths, it will be seen as yellow. If it is expanded even more, it will cover more wavelengths than the yellow semichrome does, approaching white, until it is reached when the full spectrum is emitted or reflected. The described process is called "cumulation". Cumulation can be started at either end of the visible spectrum (we just described cumulation starting from the red end of the spectrum, generating the "warm" sharp edge), cumulation starting at the violet end of the spectrum will generate the "cold" sharp edge.

Maximum chroma colors, semichromes, or full colors
Each hue has a maximum chroma color, also known as maximum chroma point, semichrome, or full color; there are no colors of that hue with a higher chroma. They are the most chromatic, vibrant optimal colors. Although we are, for now, unable to produce them, these are the colors that would be located in an ideal color wheel. They were called semichromes or full colors by the German chemist and philosopher Wilhelm Ostwald in the early 20th century.

If B is the complementary wavelength of wavelength A, then the straight line that connects A and B passes through the achromatic axis in a linear color space, such as LMS or CIE 1931 XYZ. If the emission or reflection spectrum of a color is 1 (100%) for all the wavelengths between A and B, and 0 for all the wavelengths on the other half of the color space, then that color is a maximum chroma color, semichrome, or full color (this is the explanation to why they were called semichromes). So maximum chroma colors are a type of optimal colors.

As explained, full colors are far from being monochromatic. If the spectral purity of a maximum chroma color is increased, its chroma will decrease, because it will approach the visible spectrum, ergo, it will approach black.

In perceptually uniform color spaces, the lightness of the full colors varies from around 30% in the violetish blue hues, to around 90% in the yellowish hues. The chroma of each maximum chroma point also varies depending of the hue; in optimal color solids plotted in perceptually uniform color spaces, semichromes like red, green, blue, violet, and magenta have a high chroma, while semichromes like yellow, orange, and cyan have a slightly lower chroma.

In color spaces, such as the HSL color space, the maximum chroma colors are located around the equator at the periphery of the color sphere. This makes color solids with a spherical shape inherently non-perceptually uniform, since they imply that all full colors have a lightness of 50%, when, as humans perceive them, there are full colors with a lightness from around 30% to around 90%. A perceptually uniform color solid has an irregular shape.

Cultural perspective
The meanings and associations of colors can play a major role in works of art, including literature.

Associations
Individual colors have a variety of cultural associations such as national colors (in general described in individual color articles and color symbolism). The field of color psychology attempts to identify the effects of color on human emotion and activity. Chromotherapy is a form of alternative medicine attributed to various Eastern traditions. Colors have different associations in different countries and cultures.

Different colors have been demonstrated to have effects on cognition. For example, researchers at the University of Linz in Austria demonstrated that the color red significantly decreases cognitive functioning in men. The combination of the colors red and yellow together can induce hunger, which has been capitalized on by a number of chain restaurants.

Color plays a role in memory development too. A photograph that is in black and white is slightly less memorable than one in color. Studies also show that wearing bright colors makes you more memorable to people you meet.

Terminology
Colors vary in several different ways, including hue (shades of red, orange, yellow, green, blue, and violet, etc), saturation, brightness. Some color words are derived from the name of an object of that color, such as "orange" or "salmon", while others are abstract, like "red".

In the 1969 study Basic Color Terms: Their Universality and Evolution, Brent Berlin and Paul Kay describe a pattern in naming "basic" colors (like "red" but not "red-orange" or "dark red" or "blood red", which are "shades" of red). All languages that have two "basic" color names distinguish dark/cool colors from bright/warm colors. The next colors to be distinguished are usually red and then yellow or green. All languages with six "basic" colors include black, white, red, green, blue, and yellow. The pattern holds up to a set of twelve: black, gray, white, pink, red, orange, yellow, green, blue, purple, brown, and azure (distinct from blue in Russian and Italian, but not English).

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Computation

A computation is any type of arithmetic or non-arithmetic calculation that is well-defined. Common examples of computation are mathematical equation solving and the execution of computer algorithms.

Mechanical or electronic devices (or, historically, people) that perform computations are known as computers. Computer science is a field that involves the study of computation.

Introduction
The notion that mathematical statements should be 'well-defined' had been argued by mathematicians since at least the 1600s, but agreement on a suitable definition proved elusive. A candidate definition was proposed independently by several mathematicians in the 1930s. The best-known variant was formalised by the mathematician Alan Turing, who defined a well-defined statement or calculation as any statement that could be expressed in terms of the initialisation parameters of a Turing machine. Other (mathematically equivalent) definitions include Alonzo Church's lambda-definability, Herbrand-Gödel-Kleene's general recursiveness and Emil Post's 1-definability.

Today, any formal statement or calculation that exhibits this quality of well-definedness is termed computable, while the statement or calculation itself is referred to as a computation.

Turing's definition apportioned "well-definedness" to a very large class of mathematical statements, including all well-formed algebraic statements, and all statements written in modern computer programming languages.

Despite the widespread uptake of this definition, there are some mathematical concepts that have no well-defined characterisation under this definition. This includes the halting problem and the busy beaver game. It remains an open question as to whether there exists a more powerful definition of 'well-defined' that is able to capture both computable and 'non-computable' statements.

Some examples of mathematical statements that are computable include:
 * All statements characterised in modern programming languages, including C++, Python, and Java.
 * All calculations carried by an electronic computer, calculator or abacus.
 * All calculations carried out on an analytical engine.
 * All calculations carried out on a Turing Machine.
 * The majority of mathematical statements and calculations given in maths textbooks.

Some examples of mathematical statements that are not computable include:

 * Calculations or statements which are ill-defined, such that they cannot be unambiguously encoded into a Turing machine: ("Paul loves me twice as much as Joe").
 * Problem statements which do appear to be well-defined, but for which it can be proved that no Turing machine exists to solve them (such as the halting problem).

The Physical process of computation
Computation can be seen as a purely physical process occurring inside a closed physical system called a computer. Turing's 1937 proof, On Computable Numbers, with an Application to the Entscheidungsproblem, demonstrated that there is a formal equivalence between computable statements and particular physical systems, commonly called computers. Examples of such physical systems are: Turing machines, human mathematicians following strict rules, digital computers, mechanical computers, analog computers and others.

The mapping account
An alternative account of computation is found throughout the works of Hilary Putnam and others. Peter Godfrey-Smith has dubbed this the "simple mapping account." Gualtiero Piccinini's summary of this account states that a physical system can be said to perform a specific computation when there is a mapping between the state of that system and the computation such that the "microphysical states [of the system] mirror the state transitions between the computational states."

The semantic account
Philosophers such as Jerry Fodor have suggested various accounts of computation with the restriction that semantic content be a necessary condition for computation (that is, what differentiates an arbitrary physical system from a computing system is that the operands of the computation represent something). This notion attempts to prevent the logical abstraction of the mapping account of pancomputationalism, the idea that everything can be said to be computing everything.

The mechanistic account
Gualtiero Piccinini proposes an account of computation based on mechanical philosophy. It states that physical computing systems are types of mechanisms that, by design, perform physical computation, or the manipulation (by a functional mechanism) of a "medium-independent" vehicle according to a rule. "Medium-independence" requires that the property can be instantiated by multiple realizers and multiple mechanisms, and that the inputs and outputs of the mechanism also be multiply realizable. In short, medium-independence allows for the use of physical variables with properties other than voltage (as in typical digital computers); this is imperative in considering other types of computation, such as that which occurs in the brain or in a quantum computer. A rule, in this sense, provides a mapping among inputs, outputs, and internal states of the physical computing system.

Mathematical models
In the theory of computation, a diversity of mathematical models of computation has been developed. Typical mathematical models of computers are the following: Giunti calls the models studied by computation theory computational systems, and he argues that all of them are mathematical dynamical systems with discrete time and discrete state space. He maintains that a computational system is a complex object which consists of three parts. First, a mathematical dynamical system $$DS$$ with discrete time and discrete state space; second, a computational setup $$H=\left(F, B_F\right)$$, which is made up of a theoretical part $$F$$, and a real part $$B_F$$; third, an interpretation $$I_{DS,H}$$, which links the dynamical system $$DS$$ with the setup $$H$$.
 * State models including Turing machine, pushdown automaton, finite state automaton, and PRAM
 * Functional models including lambda calculus
 * Logical models including logic programming
 * Concurrent models including actor model and process calculi

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Computation


text:
Talk:Computer jargon

Untitled
''Any chance Eric Raymond could be persuaded to add the content to Wikipedia? Just a thought.''

''I doubt he'd do it personally, but the text is already in the public domain. http://www.tuxedo.org/~esr/jargon/mirroring.html'' -- GWO

Someone with server side access to Wikipedia (Larry Sanger? Jimbo Wales?) could easily add the whole thing automatically; but those people would probably say 'Wikipedia is not a dictionary'.

On the other hand, some of the material from the appendicies could probably be incorporated in an article on hackers. -- Simon J Kissane

A lot of the content here is already available at List of computing and IT abbreviations, which I've worked on. I think these two articles should probably cross-link. Dataphile 07:57, Aug 14, 2004 (UTC)

Should the search enquiry “computerese” be redirected to this page?
I wonder why the search enquiry computerese doesn’t redirect to the page about “computer jargon”? It just returns as Article Not Found.

not jargon
A lot of these entries are not jargon. Things like: Hypertext, Interface, LIFO are the standard technical terms for the things they apply to - they are terminology. Jargon should be reserved for stuff like ID10T error, KISS, which are informal usage. --Pfold 14:15, 23 March 2006 (UTC)

Also
There is a secret society called The Secret Society of Jargon

title: Talk:Computer jargon


text:
Clown

A clown is a person who performs physical comedy and arts in an open-ended fashion, typically while wearing distinct makeup or costuming and reversing folkway-norms. The art of performing as a clown is known as clowning or buffoonery, and the term "clown" may be used synonymously with predecessors like jester, buffoon, joker, fool, or harlequin. Clowns have a diverse tradition with significant variations in costume and performance. The most recognisable clowns are those that commonly perform in the circus, characterized by colorful wigs, red noses, and oversized shoes. However, clowns have also played roles in theater and folklore, like the court jesters of the Middle Ages and the jesters and ritual clowns of various indigenous cultures. Their performances can elicit a range of emotions, from humor and laughter to fear and discomfort, reflecting complex societal and psychological dimensions. Through the centuries, clowns have continued to play significant roles in society, evolving alongside changing cultural norms and artistic expressions.

History
The most ancient clowns have been found in the Fifth Dynasty of Egypt, around 2400 BC. Unlike court jesters, clowns have traditionally served a socio-religious and psychological role, and traditionally the roles of priest and clown have been held by the same persons. Peter Berger writes, "It seems plausible that folly and fools, like religion and magic, meet some deeply rooted needs in human society." For this reason, clowning is often considered an important part of training as a physical performance discipline, partly because tricky subject matter can be dealt with, but also because it requires a high level of risk and play in the performer.

In anthropology, the term clown has been extended to comparable jester or fool characters in non-Western cultures. A society in which such clowns have an important position are termed clown societies, and a clown character involved in a religious or ritual capacity is known as a ritual clown.

Many native tribes have a history of clowning, such as the Pueblo clown of the Kachina culture. A Heyoka is an individual in Lakota and Dakota cultures who lives outside the constraints of normal cultural roles, playing the role of a backwards clown by doing everything in reverse. The Heyoka role is sometimes best filled by a Winkte. Canadian First Nations also feature jester-like ritual performers, translated by one Anishinaabe activist as "Harlequins", though the exact nature of their role is kept secret from non-members of the tribe into the present day.

The Canadian clowning method developed by Richard Pochinko and furthered by his former apprentice, Sue Morrison, combines European and Native American clowning techniques. In this tradition, masks are made of clay while the creator's eyes are closed. A mask is made for each direction of the medicine wheel. During this process, the clown creates a personal mythology that explores their personal experiences.

The circus clown tradition developed out of earlier comedic roles in theatre or Varieté shows during the 19th to mid 20th centuries. This recognizable character features outlandish costumes, distinctive makeup, colorful wigs, exaggerated footwear, and colorful clothing, with the style generally being designed to entertain large audiences.

The first mainstream clown role was portrayed by Joseph Grimaldi (who also created the traditional whiteface make-up design). In the early 1800s, he expanded the role of Clown in the harlequinade that formed part of British pantomimes, notably at the Theatre Royal, Drury Lane and the Sadler's Wells and Covent Garden theatres. He became so dominant on the London comic stage that harlequinade Clowns became known as "Joey", and both the nickname and Grimaldi's whiteface make-up design are still used by other clowns.

The comedy that clowns perform is usually in the role of a fool whose everyday actions and tasks become extraordinary—and for whom the ridiculous, for a short while, becomes ordinary. This style of comedy has a long history in many countries and cultures across the world. Some writers have argued that due to the widespread use of such comedy and its long history it is a need that is part of the human condition.

The modern clowning school of comedy in the 21st century diverged from white-face clown tradition, with more of an emphasis on personal vulnerability and heightened sexuality.

Origin
The clown character developed out of the zanni rustic fool characters of the early modern commedia dell'arte, which were themselves directly based on the rustic fool characters of ancient Greek and Roman theatre. Rustic buffoon characters in Classical Greek theater were known as sklêro-paiktês (from paizein: to play (like a child)) or deikeliktas, besides other generic terms for rustic or peasant. In Roman theater, a term for clown was fossor, literally digger; labourer.

The English word clown was first recorded c. 1560 (as clowne, cloyne) in the generic meaning rustic, boor, peasant. The origin of the word is uncertain, perhaps from a Scandinavian word cognate with clumsy. It is in this sense that Clown is used as the name of fool characters in Shakespeare's Othello and The Winter's Tale. The sense of clown as referring to a professional or habitual fool or jester developed soon after 1600, based on Elizabethan rustic fool characters such as Shakespeare's.

The harlequinade developed in England in the 17th century, inspired by Arlecchino and the commedia dell'arte. It was here that Clown came into use as the given name of a stock character. Originally a foil for Harlequin's slyness and adroit nature, Clown was a buffoon or bumpkin fool who resembled less a jester than a comical idiot. He was a lower class character dressed in tattered servants' garb.

The now-classical features of the clown character were developed in the early 1800s by Joseph Grimaldi, who played Clown in Charles Dibdin's 1800 pantomime Peter Wilkins: or Harlequin in the Flying World at Sadler's Wells Theatre, where Grimaldi built the character up into the central figure of the harlequinade.

Modern circuses
The circus clown developed in the 19th century. The modern circus derives from Philip Astley's London riding school, which opened in 1768. Astley added a clown to his shows to amuse the spectators between equestrian sequences. American comedian George L. Fox became known for his clown role, directly inspired by Grimaldi, in the 1860s. Tom Belling senior (1843–1900) developed the red clown or Auguste (Dummer August) character c. 1870, acting as a foil for the more sophisticated white clown. Belling worked for Circus Renz in Vienna. Belling's costume became the template for the modern stock character of circus or children's clown, based on a lower class or hobo character, with red nose, white makeup around the eyes and mouth, and oversized clothes and shoes. The clown character as developed by the late 19th century is reflected in Ruggero Leoncavallo's 1892 opera Pagliacci (Clowns). Belling's Auguste character was further popularized by Nicolai Poliakoff's Coco in the 1920s to 1930s.

The English word clown was borrowed, along with the circus clown act, by many other languages, such as French clown, Russian (and other Slavic languages) кло́ун, Greek κλόουν, Danish/Norwegian klovn, Romanian clovn etc.

Italian retains Pagliaccio, a Commedia dell'arte zanni character, and derivations of the Italian term are found in other Romance languages, such as French Paillasse, Spanish payaso, Catalan/Galician pallasso, Portuguese palhaço, Greek παλιάτσος, Turkish palyaço, German Pajass (via French) Yiddish פּאַיאַץ (payats), Russian пая́ц, Romanian paiață.

20th-century North America
In the early 20th century, with the disappearance of the rustic simpleton or village idiot character of everyday experience, North American circuses developed characters such as the tramp or hobo. Examples include Marceline Orbes, who performed at the Hippodrome Theater (1905), Charlie Chaplin's The Tramp (1914), and Emmett Kelly's Weary Willie based on hobos of the Depression era. Another influential tramp character was played by Otto Griebling during the 1930s to 1950s. Red Skelton's Dodo the Clown in The Clown (1953), depicts the circus clown as a tragicomic stock character, "a funny man with a drinking problem".

In the United States, Bozo the Clown was an influential Auguste character since the late 1950s. The Bozo Show premiered in 1960 and appeared nationally on cable television in 1978. McDonald's derived its mascot clown, Ronald McDonald, from the Bozo character in the 1960s. Willard Scott, who had played Bozo during 1959–1962, performed as the mascot in 1963 television spots. The McDonald's trademark application for the character dates to 1967.

Based on the Bozo template, the US custom of birthday clown, private contractors who offer to perform as clowns at children's parties, developed in the 1960s to 1970s. The strong association of the (Bozo-derived) clown character with children's entertainment as it has developed since the 1960s also gave rise to Clown Care or hospital clowning in children's hospitals by the mid-1980s. Clowns of America International (established 1984) and World Clown Association (established 1987) are associations of semi-professionals and professional performers.

The shift of the Auguste or red clown character from his role as a foil for the white in circus or pantomime shows to a Bozo-derived standalone character in children's entertainment by the 1980s also gave rise to the evil clown character, with the attraction of clowns for small children being based in their fundamentally threatening or frightening nature. The fear of clowns, particularly circus clowns, has become known by the term "coulrophobia."

Types
There are different types of clowns portrayed around the world. They include

 * Auguste
 * Blackface
 * Buffoon
 * Harlequin
 * Jester
 * Mime artist
 * Pierrot
 * Pueblo
 * Rodeo clown
 * Tramp
 * Whiteface

Pierrot and Harlequin
The classical pairing of the White Clown with Auguste in modern tradition has a precedent in the pairing of Pierrot and Harlequin in the Commedia dell'arte. Originally, Harlequin's role was that of a light-hearted, nimble and astute servant, paired with the sterner and melancholic Pierrot.

In the 18th-century English Harlequinade, Harlequin was now paired with Clown. As developed by Joseph Grimaldi around 1800, Clown became the mischievous and brutish foil for the more sophisticated Harlequin, who became more of a romantic character. The most influential such pair in Victorian England were the Payne Brothers, active during the 1860s and 1870s.

White and Auguste

The white clown, or clown blanc in French, is a sophisticated character, as opposed to the clumsy Auguste. The two types are also distinguished as the sad clown (blanc) and happy clown (Auguste).

The Auguste face base makeup color is a variation of pink, red, or tan rather than white. Features are exaggerated in size, and are typically red and black in color. The mouth is thickly outlined with white (called the muzzle) as are the eyes. Appropriate to the character, the Auguste can be dressed in either well-fitted garb or a costume that does not fit – oversize or too small, either is appropriate. Bold colors, large prints or patterns, and suspenders often characterize Auguste costumes.

The Auguste character-type is often an anarchist, a joker, or a fool. He is clever and has much lower status than the whiteface. Classically the whiteface character instructs the Auguste character to perform his bidding. The Auguste has a hard time performing a given task, which leads to funny situations. Sometimes the Auguste plays the role of an anarchist and purposefully has trouble following the whiteface's directions. Sometimes the Auguste is confused or is foolish and makes errors less deliberately.

The contra-auguste plays the role of the mediator between the white clown and the Auguste character. He has a lower status than the white clown but a higher status than the Auguste. He aspires to be more like the white clown and often mimics everything the white clown does to try to gain approval. If there is a contra-auguste character, he often is instructed by the whiteface to correct the Auguste when he is doing something wrong.

There are two major types of clowns with whiteface makeup: The classic white clown is derived from the Pierrot character. His makeup is white, usually with facial features such as eyebrows emphasized in black. He is the more intelligent and sophisticated clown, contrasting with the rude or grotesque Auguste types. Francesco Caroli and Glenn "Frosty" Little are examples of this type. The second type of whiteface is the buffoonish clown of the Bozo type, known as Comedy or Grotesque Whiteface. This type has grotesquely emphasized features, especially a red nose and red mouth, often with partial (mostly red) hair. In the comedic partnership of Abbott and Costello, Bud Abbot would have been the classic whiteface and Lou Costello the comedy whiteface or Auguste.

Traditionally, the whiteface clown uses clown white makeup to cover the entire face and neck, leaving none of the underlying natural skin visible. In the European whiteface makeup, the ears are painted red.

Whiteface makeup was originally designed by Joseph Grimaldi in 1801. He began by painting a white base over his face, neck and chest before adding red triangles on the cheeks, thick eyebrows and large red lips set in a mischievous grin. Grimaldi's design is used by many modern clowns. According to Grimaldi's biographer Andrew McConnell Stott, it was one of the most important theatrical designs of the 1800s.

America's first great whiteface clown was stage star George "G.L." Fox. Inspired by Grimaldi, Fox popularised the Humpty Dumpty stories throughout the U.S. in the 1860s.

In horror
The scary clown, also known as the evil clown or killer clown, is a subversion of the traditional comic clown character, in which the playful trope is instead depicted in a more disturbing nature through the use of horror elements and dark humor. The character can be seen as playing on the sense of unease felt by those with coulrophobia, the fear of clowns. The modern archetype of the evil clown was popularized by DC Comics character the Joker starting in 1940 and again by Pennywise in Stephen King's novel It, which introduced the fear of an evil clown to a modern audience. In the novel, the eponymous character is a pan-dimensional monster which feeds mainly on children by luring them in the form of a clown, named "Pennywise", and then assuming the shape of whatever the victim fears the most.

Character
The character clown adopts an eccentric character of some type, such as a butcher, a baker, a policeman, a housewife or hobo. Prime examples of this type of clown are the circus tramps Otto Griebling and Emmett Kelly. Red Skelton, Harold Lloyd, Buster Keaton, Charlie Chaplin, Rowan Atkinson and Sacha Baron Cohen would all fit the definition of a character clown.

The character clown makeup is a comic slant on the standard human face. Their makeup starts with a flesh tone base and may make use of anything from glasses, mustaches and beards to freckles, warts, big ears or strange haircuts.

The most prevalent character clown in the American circus is the hobo, tramp or bum clown. There are subtle differences in the American character clown types. The primary differences among these clown types is attitude. According to American circus expert Hovey Burgess, they are:
 * The Hobo: Migratory and finds work where he travels. Down on his luck but maintains a positive attitude.
 * The Tramp: Migratory and does not work where he travels. Down on his luck and depressed about his situation.
 * The Bum: Non-migratory and non-working.

Organizations
The World Clown Association is a worldwide organization for clowns, jugglers, magicians, and face painters. It holds an annual convention, mainly in the United States.

Clowns of America International is a Minnesota-based non-profit clown arts membership organization which aims "to share, educate, and act as a gathering place for serious minded amateurs, semiprofessionals, and professional clowns".

Clowns International is a British clowning organisation dating back to the 1940s. It is responsible for the Clown Egg Register.

Roles and skills
In the circus, a clown might perform other circus roles or skills. Clowns may perform such skills as tightrope, juggling, unicycling, Master of Ceremonies, or ride an animal. Clowns may also "sit in" with the orchestra. Other circus performers may also temporarily stand in for a clown and perform their skills in clown costume.

Frameworks
Frameworks are the general outline of an act that clowns use to help them build out an act. Frameworks can be loose, including only a general beginning and ending to the act, leaving it up to the clown's creativity to fill in the rest, or at the other extreme a fully developed script that allows very little room for creativity.

Shows are the overall production that a clown is a part of, it may or may not include elements other than clowning, such as in a circus show. In a circus context, clown shows are typically made up of some combination of entrées, side dishes, clown stops, track gags, gags and bits.

Gags, bits and business

 * Business – the individual motions the clown uses, often used to express the clown's character.
 * Gag – very short piece of clown comedy that, when repeated within a bit or routine, may become a running gag. Gags are, loosely, the jokes clowns play on each other. A gag may have a beginning, a middle, and an end – or may not. Gags can also refer to the prop stunts/tricks or the stunts that clowns use, such as a squirting flower.
 * Bit – the clown's sketch or routine, made up of one or more gags either worked out and timed before going on stage, or impromptu bits composed of familiar improvisational material

Menu

 * Entrée — clowning acts lasting 5–10 minutes. Typically made up of various gags and bits, usually within a clowning framework. Entrées almost always end with a blow-off — the comedic ending of a show segment, bit, gag, stunt, or routine.
 * Side dish — shorter feature act. Side dishes are essentially shorter versions of the entrée, typically lasting 1–3 minutes. Typically made up of various gags and bits, side dishes are usually within a clowning framework. Side dishes almost always end with a blow-off.

Interludes
Clown Stops or interludes are the brief appearances of clowns in a circus while the props and rigging are changed. These are typically made up of a few gags or several bits. Clown stops will always have a beginning, a middle, and an end to them, invariably culminating in a blow-off. These are also called reprises or run-ins by many, and in today's circus they are an art form in themselves. Originally they were bits of business usually parodying the preceding act. If for instance there had been a tightrope walker the reprise would involve two chairs with a piece of rope between and the clown trying to imitate the artiste by trying to walk between them, with the resulting falls and cascades bringing laughter from the audience. Today, interludes are far more complex, and in many modern shows the clowning is a thread that links the whole show together.

Prop stunts
Among the more well-known clown stunts are: squirting flower; the too-many-clowns-coming-out-of-a-tiny-car stunt; doing just about anything with a rubber chicken, tripping over one's own feet (or an air pocket or imaginary blemish in the floor), or riding any number of ridiculous vehicles or clown bicycles. Individual prop stunts are generally considered individual bits.

title: Clown


text:
Coffea

Coffea is a genus of flowering plants in the family Rubiaceae. Coffea species are shrubs or small trees native to tropical and southern Africa and tropical Asia. The seeds of some species, called coffee beans, are used to flavor various beverages and products. The fruits, like the seeds, contain a large amount of caffeine, and have a distinct sweet taste.

The plant ranks as one of the world's most valuable and widely traded commodity crops and is an important export product of several countries, including those in Central and South America, the Caribbean and Africa. The coffee trade relies heavily on two of the over 120 species, Coffea arabica (commonly known simply as "Arabica"), which accounts for 60–80% of the world's coffee production, and Coffea canephora (known as "Robusta"), which accounts for about 20–40%. Both coffee species are vulnerable to shifts, caused by climate change, in their growing zones, which are likely to result in a decline in production in some of the most important growing regions.

Cultivation and use
There are over 130 species of Coffea, which is grown from seed. The two most popular are Coffea arabica (commonly known simply as "Arabica"), which accounts for 60–80% of the world's coffee production, and Coffea canephora (known as "Robusta"), which accounts for about 20–40%. C. arabica is preferred for its sweeter taste, while C. canephora has a higher caffeine content. C. arabica has its origins in the highlands of Ethiopia and the Boma Plateau of Sudan, and came about as the result of a hybrid between C. canephora and C. eugenioides.

The trees produce edible red or purple fruits that are either epigynous berries or indehiscent drupes. The fruit is often referred to as a "coffee cherry", and it contains two seeds, called "coffee beans". Despite these terms, coffee is neither a true cherry (the fruit of certain species in the genus Prunus) nor a true bean (seeds from plants in the family Fabaceae).

In any coffee crop, about 5–10% of fruits contain only a single bean. Called a peaberry, it is smaller and rounder than a normal coffee bean.

When grown in the tropics, coffee is a vigorous bush or small tree that usually grows to a height of 3 - 3.5 m. Most commonly cultivated coffee species grow best at high elevations, but do not tolerate freezing temperatures.

The Coffea arabica tree grows fruit after three to five years, producing for an average of 50 to 60 years, though up to 100 years is possible. The white flowers are highly scented. The fruit takes about nine months to ripen.

Ecology
The caffeine in coffee beans serves as a toxic substance that protects against insects and other pests, a form of natural plant defense against herbivory. Caffeine simultaneously attracts pollinators, specifically honeybees, by creating an olfactory memory that signals bees to return to the plant's flowers. Not all Coffea species contain caffeine, and the earliest species had little or no caffeine content. Caffeine has evolved independently in multiple lineages of Coffea in Africa, perhaps in response to high pest predation in the humid environments of West-Central Africa. Caffeine has also evolved independently in the more distantly related genera Theobroma (cacao) and Camellia (tea). This suggests that caffeine production is an adaptive trait in coffee and plant evolution. The fruit and leaves also contain caffeine, and can be used to make coffee cherry tea and coffee-leaf tea. The fruit is also used in many brands of soft drink as well as pre-packaged teas.

Several insect pests affect coffee production, including the coffee borer beetle (Hypothenemus hampei) and the coffee leafminer (Leucoptera caffeina).

Coffee is used as a food plant by the larvae of some Lepidoptera (butterfly and moth) species, Dalcera abrasa, turnip moth and some members of the genus Endoclita, including E. damor and E. malabaricus.

Research
New species of Coffea are still being identified in the 2000s. In 2008 and 2009, researchers from the Royal Botanic Gardens, Kew, named seven from the mountains of northern Madagascar, including C. ambongensis, C. boinensis, C. labatii, C. pterocarpa, C. bissetiae, and C. namorokensis.

In 2008, two new species were discovered in Cameroon: Coffea charrieriana, which is caffeine-free, and Coffea anthonyi. By crossing the new species with other known coffees, two new features might be introduced to cultivated coffee plants: beans without caffeine and self-pollination.

In 2011, Coffea absorbed the twenty species of the former genus Psilanthus due to the morphological and genetic similarities between the two genera. Historically, the two have been considered distinct genera due to differences in the length of the corolla tube and the anther arrangement: Coffea with a short corolla tube and exserted style and anthers; Psilanthus with a long corolla tube and included anthers. However, these characteristics were not present in all species of either respective genus, making the two genera overwhelmingly similar in both morphology and genetic sequence. This transfer expanded Coffea from 104 species to 124, and extended its native distribution to tropical Asia and Australasia.

The coffee genome was published in 2014, with more than 25,000 genes identified. This revealed that coffee plants make caffeine using a different set of genes from those found in tea, cacao and other such plants.

A robust and almost fully resolved phylogeny of the entire genus was published in 2017. In addition to resolving the relationships of Coffea species, this study's results suggest Africa or Asia as the likely ancestral origin of Coffea and point to several independent radiations across Africa, Asia, and the Western Indian Ocean Islands.

In 2020, a technique of DNA fingerprinting, or genetic authentication of plant material, was proven effective for coffee. For the study, scientists used DNA extraction and SSR marker analysis. This technique or similar ones may allow for several improvements to coffee production such as improved information for farmers as to the susceptibility of their coffee plants to pests and disease, a professionalized coffee seed system, and transparency and traceability for buyers of green, un-roasted coffee.

Species
As of May 2024, Plants of the World Online includes:

 * 1) Coffea abbayesii J.-F.Leroy
 * 2) Coffea affinis De Wild.
 * 3) Coffea alleizettii Dubard
 * 4) Coffea ambanjensis J.-F.Leroy
 * 5) Coffea ambongenis J.-F.Leroy ex A.P.Davis
 * 6) Coffea andrambovatensis J.-F.Leroy
 * 7) Coffea ankaranensis J.-F.Leroy ex A.P.Davis
 * 8) Coffea anthonyi Stoff. & F.Anthony
 * 9) Coffea arabica L.
 * 10) Coffea arenesiana J.-F.Leroy
 * 11) Coffea augagneurii Dubard
 * 12) Coffea bakossii Cheek & Bridson
 * 13) Coffea benghalensis B.Heyne ex Schult.
 * 14) Coffea bertrandii A.Chev.
 * 15) Coffea betamponensis Portères & J.-F.Leroy
 * 16) Coffea bissetiae A.P.Davis & Rakotonas.
 * 17) Coffea boinensis A.P.Davis & Rakotonas.
 * 18) Coffea boiviniana A.P.Davis & Rakotonas.
 * 19) Coffea bonnieri Dubard
 * 20) Coffea brassii (J.-F.Leroy) A.P.Davis
 * 21) Coffea brevipes Hiern
 * 22) Coffea bridsoniae A.P.Davis & Mvungi
 * 23) Coffea buxifolia A.Chev.
 * 24) Coffea callmanderi
 * 25) Coffea canephora ("Coffea robusta") Pierre ex A.Froehner
 * 26) Coffea carrissoi A.Chev.
 * 27) Coffea charrieriana Stoff. & F.Anthony
 * 28) Coffea cochinchinensis Pierre ex Pit.
 * 29) Coffea commersoniana (Baill.) A.Chev.
 * 30) Coffea congensis A.Froehner
 * 31) Coffea costatifructa Bridson
 * 32) Coffea coursiana J.-F.Leroy
 * 33) Coffea dactylifera Robbr. & Stoff.
 * 34) Coffea darainensis
 * 35) Coffea decaryana J.-F.Leroy
 * 36) Coffea dubardii Jum.
 * 37) Coffea ebracteolata (Hiern) Brenan
 * 38) Coffea eugenioides S.Moore
 * 39) Coffea fadenii Bridson
 * 40) Coffea farafanganensis J.-F.Leroy
 * 41) Coffea floresiana Boerl.
 * 42) Coffea fotsoana Stoff. & Sonké
 * 43) Coffea fragilis J.-F.Leroy
 * 44) Coffea fragrans Wall. ex Hook.f.
 * 45) Coffea gallienii Dubard
 * 46) Coffea grevei Drake ex A.Chev.
 * 47) Coffea heimii J.-F.Leroy
 * 48) Coffea × heterocalyx
 * 49) Coffea homollei J.-F.Leroy
 * 50) Coffea horsfieldiana Miq.
 * 51) Coffea humbertii J.-F.Leroy
 * 52) Coffea humblotiana Baill.
 * 53) Coffea humilis A.Chev.
 * 54) Coffea jumellei J.-F.Leroy
 * 55) Coffea kalobinonensis
 * 56) Coffea kapakata (A.Chev.) Bridson
 * 57) Coffea kianjavatensis J.-F.Leroy
 * 58) Coffea kihansiensis A.P.Davis & Mvungi
 * 59) Coffea kimbozensis Bridson
 * 60) Coffea kivuensis Lebrun
 * 61) Coffea labatii A.P.Davis & Rakotonas.
 * 62) Coffea lancifolia A.Chev.
 * 63) Coffea lebruniana Germ. & Kester
 * 64) Coffea leonimontana Stoff.
 * 65) Coffea leroyi A.P.Davis
 * 66) Coffea liaudii J.-F.Leroy ex A.P.Davis
 * 67) Coffea liberica Hiern
 * 68) Coffea ligustroides S.Moore
 * 69) Coffea littoralis A.P.Davis & Rakotonas.
 * 70) Coffea lulandoensis Bridson
 * 71) Coffea mabesae (Elmer) J.-F.Leroy
 * 72) Coffea macrocarpa A.Rich.
 * 73) Coffea madurensis Teijsm. & Binn. ex Koord.
 * 74) Coffea magnistipula Stoff. & Robbr.
 * 75) Coffea malabarica (Sivar., Biju & P.Mathew) A.P.Davis
 * 76) Coffea mangoroensis Portères
 * 77) Coffea mannii (Hook.f.) A.P.Davis
 * 78) Coffea manombensis A.P.Davis
 * 79) Coffea mapiana Sonké, Nguembou & A.P.Davis
 * 80) Coffea mauritiana Lam.
 * 81) Coffea mayombensis A.Chev.
 * 82) Coffea mcphersonii A.P.Davis & Rakotonas.
 * 83) Coffea melanocarpa Welw. ex Hiern
 * 84) Coffea merguensis Ridl.
 * 85) Coffea microdubardii
 * 86) Coffea millotii J.-F.Leroy
 * 87) Coffea minutiflora A.P.Davis & Rakotonas.
 * 88) Coffea mogenetii Dubard
 * 89) Coffea mongensis Bridson
 * 90) Coffea montekupensis Stoff.
 * 91) Coffea montis-sacri A.P.Davis
 * 92) Coffea moratii J.-F.Leroy ex A.P.Davis & Rakotonas.
 * 93) Coffea mufindiensis Hutch. ex Bridson
 * 94) Coffea myrtifolia (A.Rich. ex DC.) J.-F.Leroy
 * 95) Coffea namorokensis A.P.Davis & Rakotonas.
 * 96) Coffea neobridsoniae A.P.Davis
 * 97) Coffea neoleroyi A.P.Davis
 * 98) Coffea perrieri Drake ex Jum. & H.Perrier
 * 99) Coffea pervilleana (Baill.) Drake
 * 100) Coffea pocsii Bridson
 * 101) Coffea pseudozanguebariae Bridson
 * 102) Coffea pterocarpa A.P.Davis & Rakotonas.
 * 103) Coffea pustulata
 * 104) Coffea racemosa Lour.
 * 105) Coffea rakotonasoloi A.P.Davis
 * 106) Coffea ratsimamangae J.-F.Leroy ex A.P.Davis & Rakotonas.
 * 107) Coffea resinosa (Hook.f.) Radlk.
 * 108) Coffea rhamnifolia (Chiov.) Bridson
 * 109) Coffea richardii J.-F.Leroy
 * 110) Coffea rizetiana
 * 111) Coffea rupicola
 * 112) Coffea sahafaryensis J.-F.Leroy
 * 113) Coffea sakarahae J.-F.Leroy
 * 114) Coffea salvatrix Swynn. & Philipson
 * 115) Coffea sambavensis J.-F.Leroy ex A.P.Davis & Rakotonas.
 * 116) Coffea sapinii (De Wild.) A.P.Davis
 * 117) Coffea schliebenii Bridson
 * 118) Coffea semsei (Bridson) A.P.Davis
 * 119) Coffea sessiliflora Bridson
 * 120) Coffea stenophylla G.Don
 * 121) Coffea tetragona Jum. & H.Perrier
 * 122) Coffea togoensis A.Chev.
 * 123) Coffea toshii A.P.Davis & Rakotonas.
 * 124) Coffea travancorensis Wight & Arn.
 * 125) Coffea tricalysioides J.-F.Leroy
 * 126) Coffea tsirananae J.-F.Leroy
 * 127) Coffea vatovavyensis J.-F.Leroy
 * 128) Coffea vavateninensis J.-F.Leroy
 * 129) Coffea vianneyi J.-F.Leroy
 * 130) Coffea vohemarensis A.P.Davis & Rakotonas.
 * 131) Coffea wightiana Wall. ex Wight & Arn.
 * 132) Coffea zanguebariae Lour.

title: Coffea