CMY
A subtractive, Trichromatic Color Model that is used to describe the mixing of colors of pigments (not light sources). Cyan, Magenta and Yellow are the Primary Colors and are added together in various ways to reproduce a broad array of colors. The main purpose of the CMY color model is for color printing.
CMY does not define the Chromaticities of the cyan/magenta/yellow Primaries Colorimetrically, so mixed colors are always relative to the primaries (and thus definitions defined in CYM are pigment dependent). When the exact chromaticities of the primary pigments are defined, the color model then becomes a Color Space (which are pigment independent).
When the pigments are translucent (almost always true), it can be impractical to generate very dark colors with a CMY Color Space, so the CYMK Color Model is used.