The process of identifying and labelling all the separate pieces of art, copy, or film that will need to be stripped into a single flat or that will be printed on the same page. Keying typically involves ensuring that both a continuous tone image or halftone supplied separately and the "holes" on the mechanical or negative are clearly marked, so that the stripper and/or printer inserts the correct image into the correct position.
In video production, the term keying refers to the process of overlapping or superimposing one video image on another. See Chroma-Keying and Genlock.