Glossary

Absolute color space

is a color space in which colors are unambiguous, where they do not depend on any external factors.
An example of an absolute color space is L*a*b*. A L*a*b* color defines an exact color. A L*a*b* color is abstract, but if reproduced using an accurate device and viewed in the right conditions, it should look exactly as intended.
A counter-example of a color space that is not absolute is RGB. RGB is made by mixing red, green, and blue, but these are not standardized, precise definitions. Two computer monitors or other RGB devices may show the same RGB image looking very different.
One way to think of this is that L*a*b* is a color, while RGB is a recipe; the results of mixing RGB depend on the ingredients.
A non-absolute color space can be made absolute by defining its ingredients more precisely. For instance, if the red, green, and blue colors in a monitor are measured exactly, together with other properties of the monitor, then RGB values on that monitor can now be considered as absolute.
A popular way to make a color space like RGB into an absolute color is to define an ICC profile, which contains the attributes of the RGB. This is not the only way to express an absolute color, but it is the standard in many industries.

Brightness

the same as Luminance.

Color Conversion

One absolute color can be converted to another absolute color, and back again, in general; however, each color space has its own gamut, and converting colors that lie outside that gamut will not produce correct results. There are also likely to be rounding errors, especially if the popular range of only 256 distinct values per component (8 bits) is used.
Also note that one part of the definition of an absolute color is the viewing conditions. The same color, viewed under different natural or artificial lighting conditions, will look different. Those involved professionally with color matching may have viewing rooms, lit by standardized lighting.
Occasionally, there are precise rules for converting between non-absolute color spaces. For example HLS is defined as a mapping of RGB. Both are non-absolute, but the conversion between them should maintain the same color. However, in general, converting between two non-absolute color spaces (for example, RGB to CMYK) or between absolute and non-absolute color spaces (for example, RGB to L*a*b*) is almost a meaningless concept, though there will be formulae that give roughly equivalent results.

Gamut

The gamut of a color system is the range of colors that can be displayed or printed. The spectrum of colors that can be viewed by the human eye is wider than any method of reproducing color. Among the color models used, Lab has the largest gamut and encompasses all the colors in the RGB and CMYK gamuts. The RGB gamut contains the subset of these colors that can be viewed on the computer or television monitor (which emits red, green, and blue light). Some colors, such as pure cyan or pure yellow, can't be displayed accurately on a monitor. The smallest gamut is that of the CMYK model, which consists of colors that can be printed using process-color inks. When colors that cannot be printed are displayed on the screen, they are referred to as out-of-gamut colors (that is, they are outside the CMYK gamut).

Hue

Hue is identified as the color family or color name (such as red, green, and blue). It embodies the visual sensation by which one determines if a colored area is similar to one of the perceived colors, red, yellow, green and blue, or a combination of two of them. Hue is directly linked to the color's wavelength and it values range between 0 and 360 degrees.

LAB Color

Lab images use three components to represent color, Lightness, Chroma Channel A and Chroma Channel B. A large percentage of the visible spectrum can be represented by mixing three basic components of colored light in various proportions and intensities. These components are known as the primary colors: Red, Green and Blue (RGB Color Model). When the three primary colors overlap, they create the secondary colors: Cyan, Magenta and Yellow. When combined together there is an absence of color leaving Black (CMYK Color Model).
Since the primary colors combine to create White, they are also called additive colors. Adding all the colors together creates white - that is, all the light is reflected back to the eye. Removing all the colors leaves Black -- as if you've turned off the lights. Additive colors are used for lighting, video, film recorders, and monitors. Your monitor, for example, creates color by emitting light through red, green, and blue phosphors. While the RGB Color Model depends on a light source to create color, the CMYK Color Model is based on the light-absorbing quality of ink printed on paper. As white light strikes translucent inks, a portion of the spectrum is absorbed. Color that is not absorbed is reflected back to your eye.
The LABb Model addresses the problem of the variability of color reproduction that results from the use of different monitors or different printing devices. LAB Color is designed to be device independent -- that is, it creates consistent color regardless of the specific device, such as the monitor, printer, or computer that you use to create or output the image. LAB color consists of a luminance, or Lightness component and two Chromatic components - the A component, which ranges from Green to Red, and the B component, which ranges from Blue to Yellow.

Luminance

Luminance, also called "Brightness", is the darkness (shade) or lightness (tint) of a color.

Saturation

Saturation represents how pure a color is. In other words, saturation is a measure of the purity of a color or how sharp or dull the color appears. It is also called "chroma".