The technique for mixing two images or frames together in which a colour (or a small colour range) from one image is removed (or made transparent), revealing another image behind it. For example it is commonly used for weather forecast broadcasts, where the presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background. A Blue or Green background is usually favoured as these colours are considered to be the furthest away from skin tone.
The Chroma Key process is based on the luminance key. In a luminance key, everything in the image over (or under) a set brightness level is "keyed" out and replaced by either another image, or a colour from a colour generator. See Blue Key.