A chroma key is the removal of a color (or small color range) from one image to reveal another image "behind" it. The removed color becomes transparent. This technique is also referred to as "color keying", "colour-separation overlay" ("CSO") (primarily by the BBC[1]), "greenscreen" and "bluescreen". It is commonly used for weather forecast broadcasts. 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.
