Green Screen

Green screen refers to a production technique used to make people or products look like they are somewhere they aren’t. The subject is filmed against a plain background, which is either green or blue. The background is later “keyed”. 

Keying or Chroma Keying is the process of isolating a single color value in an electronic image. Once isolated, software is used to make that value transparent, allowing another image to show through the affected area(s).