The technique (in Film & in Video) of mixing two (or more) different images together in which a specific color (typically a blue screen or green screen) is removed in order to reveal a different image behind it.  Frequently seen on Weather Reports on TV, the Meteorologist is usually seen standing in front of a Map or weather chart.  The reality is they are standing in front of a screen with a large blue or green background.  This process has been perfected to the point that today’s Special Effects companies use Greenscreens to amazing Audiences with some of Film’s most incredible visual effects.