Chroma key

A special effects technique that allows you to derive an alpha channel or matte from the green or blue background of a video clip. First you record a subject against an evenly lit green or blue background. Then, through a digital process, the colored background is made transparent so the clip can be composited against other clips, creating the illusion of a single shot. Green-screen technology is what makes weather forecasters appear to be standing against an animated map, when in reality they’re standing in front of a green wall. Also known as green-screen keying, blue-screen keying, or chroma keying.