CMYK are the initials of the 4 colors used in color printing: specifically CYAN, MAGENTA, YELLOW and BLACK (the web uses three colors: RED, GREEN, BLUE). With these 4 colors, all the print collateral you encounter on a daily basis is created: magazines, book covers, packaging, bills, illustrated bibles, you name it.
Designers throw it around constantly, and it is something so integrated into our language that often we forget the rest of the world has no idea what we are talking about. I think every designer has a moment where they refer to CMYK in a public setting, only to meet a blank stare. To designers, these initials are one of those little phrases that can identify someone as one of our own. Therefore, I sometimes think of CMYK as a sort of code word: like the password to get into the clubhouse your brother refused you access to.
To me, it also has a deeper meaning. CMYK are the four standard colors we build from, and in a sense, the prime elements of design. They are a designer’s Earth, Air, Fire and Water. It is with these four points that all possibilities spring from. They are the source of all inspiration, creativity, and craft.