Chroma key is the terminology used for the process of shooting against an even colored background [typically green or blue], which is removed electronically in post production to combine both images replacing the colored background all together revealing the combined final image.

This technique is  commonly used for Television weather forecast were the presenter appears to be standing in front of a large weather map, but the presenter is in the television Studio in front of a large blue or green background.
Elizabeth Perez TV News reporter 

Elizabeth Perez TV News Reporter and USA

President Barack Obama in a Chroma-key Photo Montage

by Joe F Cardenas, B.S.C..

The principal subject is filmed, videotape or photographed against an even colored selected background, usually blue or green. In the portions of the film, video or digital photograph which match the preselected color are replaced by the alternate background film, video or digital photograph.

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