posterization - technical definition

The effect produced when a photographic image is displayed or printed with a small number of colors or shades of gray. For example, displaying color photographs or video with 16 colors produces a visible posterization, but the images are discernible. At 256 colors, the flesh tones on color images are still mildly posterized. For realistic flesh tones, it takes 65K colors. For absolute realism, it requires 16M colors.