keying (telecommunications) is a form of modulation where the modulating signal takes one of two or more values at all times. For example: "on" or "off", "mark" or "space". The name derives from the Morse code key used for telegraph signalling.
keying (graphics) is an informal term for compositing two full frame images together, by discriminating the visual information into values of color and light.
using a key to scratch the paint on car as a form of vandalism.
keying (social theory) is the set of conventions by which a framed activity (a fight) is transformed into something patterned on this activity, but seen by participants as something quite different (play).