The difference between the printed color and the ideal color which it is supposed to represent. For example, cyan ink used in four-color process work should ideally reflect all the green and blue frequencies of light that fall on it, while it should absorb all of the red frequencies. In reality, the ink will not achieve this state of perfection.
The degree of contamination in a process colour ink or other pigments that alters its appearance from that of a perfect process colour. For example most magenta inks are contaminated with yellow, making them appear to be red, rather than purplish-red.