Refers to four single-color images generated by separating a color image into its cyan, magenta, yellow, and black components.
Separations are produced with the use of a process camera and the appropriate filters (red, green, and blue; the primary colors of light).
When a red filter is used, the red light is filtered out, leaving green and blue which combine to form the cyan image.
The green filter produces a magenta image, and so on. There are computer programs which will also accomplish this.