An alternate term for black ink used in multi-color printing. When letterpress was the dominant printing process, black was always the first ink down, the registration of all the subsequent colors being keyed to the black. In process color terminology, the "K" in CMYK stands for "key," or black. A black printer is a key plate made which includes only a range of black tonal gradations, used to enhance the contrast of the image as a whole, and improve the detail in the shadows. Black printers can either be full-scale black or half-scale black.
The term key also means, in prepress, to precisely identify page elements prepared separately but destined to be printed on the same page. See Keying (first definition).
In video production, the term key also means to insert one video image onto another. See Keying (second definition).