From The Free On-line Dictionary of Computing (8 July 2008) [foldoc]:
CMYK
<graphics> cyan, magenta, yellow, key.
A colour model that describes each colour in terms of the
quantity of each secondary colour (cyan, magenta, yellow), and
"key" (black) it contains. The CMYK system is used for
printing. For mixing of pigments, it is better to use the
secondary colours, since they mix subtractively instead of
additively. The secondary colours of light are cyan, magenta
and yellow, which correspond to the primary colours of pigment
(blue, red and yellow). In addition, although black could be
obtained by mixing these three in equal proportions, in
four-colour printing it always has its own ink. This gives
the CMYK model. The K stands for "Key' or 'blacK,' so as not
to cause confusion with the B in RGB.
Alternative colour models are RGB and HSB.
(1994-12-22)