The full form of CMYK is Cyan, Magenta, Yellow, Key (black). It’s used on Technology ,Imaging & Printing in Worldwide

CMYK color model is a subtractive color model used in printing process. CMYK refers to Cyan, Magenta, Yellow, and Key (black). Some sources say that “K” in CMYK stands for key because in four-color printing, cyan, magenta, and yellow printing plates are carefully aligned with the black plate which is the key. key is a shorthand for the printing term key plate. And some sources suggest that the “K” in CMYK comes from the last letter in “blacK” and was chosen because B already means Blue.
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In general, CMYK model is used for color printing, while the RGB (Red, Green, Blue) model is used mainly for computer displays.