Stands for "Cyan Magenta Yellow Black," or "Cyan Magenta Yellow Key."
CMYK is a color model used for printing color images. It separates colors into four channels — cyan, magenta, yellow, and black. These colors correspond to the four ink colors used in home and commercial printers.
Graphics and documents must be converted from RGB into CMYK before printing, either by image editing software or automatically by the printer driver. Some colors may appear duller after conversion to CMYK due to differences between the two models. RGB color spaces typically have a larger color gamut, since computer monitors can display more colors than mixed inks.
Unlike RGB and most other color models, CMYK is a subtractive model. All four CMYK color sliders set to 0% creates white, since that value represents the lack of ink on a blank page. Increasing the value of each color slider results in darker colors as more ink is added, until each slider at 100% results in black.
CMYK color uses black ink in addition to the three colored inks for several reasons. Black ink is cheaper than color ink, so it is more efficient to use it whenever possible. Black ink also produces a truer black than a combination of color inks due to impurities present in the colors. Creating black by mixing colors also requires that the print heads or plates for the three colors are perfectly aligned; otherwise, text, lines, and other details will blur.
NOTE: The "K" in the abbreviation represents black ink, but it actually stands for "key." This term comes from the plates used in a commercial four-color printing process. The three color ink plates create the areas of color on the page, but the black ink plate is responsible for the contrast and fine details in the resulting image. As a result, the black ink plate is known as the "key plate."