Abbreviation for Cyan, Magenta, Yellow, and Key. This is a subtractive color model used in color printing and color film. This model is based on mixing pigments of the included colors in order to make all other colors. Cyan is a blue-green; Magenta is a combination of equal levels of red and blue. The “Key” refers to a degree of black achieved by mixing the other three colors.
CMYK works through light absorption. The colors that you see are from the part of light that is not absorbed. In CMYK magenta plus yellow produces red; magenta plus cyan makes blue; cyan plus yellow generates green; and the combination of all three forms black.
In color film processing, cyan, magenta and yellow dyes are applied to film stock – frequently each on a separate layer – in the correct proportions to produce the original colors (or at least an analogy to those colors). CMYK differs from RGB, which is an additive method of creating colors by utilizing red, green, and blue light combined in various ratios. RGB is the model employed by cathode ray tubes for video.
By the way, because the “black” generated by mixing the subtractive hues is not as dense as that of a genuine black ink (one that absorbs throughout the visible spectrum), four-color printing uses black ink in addition to the subtractive primaries yellow, magenta and cyan.