Process color refers to the inks and process used when a wide range of colors are reproduced using a limited number of inks.
CMYK is the most common process ink set. Cyan, Magenta, Yellow, and black(Key) inks are combined in varying amounts to produce a reasonably wide range of colors. This is much cheaper than using a different ink for every color required and in the case of continuous-tone images like photographs, makes printing in color possible. Also referred to as "four color" process.
Color made from a typical CMYK printing process.
Process color is referred to in contrast to Spot Color processes which allow for printing of colors that "process color" cannot produce.