blue screen

Short definition from Wiktionary

Noun

1. 1
   The filmmaking technique of shooting foreground action against an evenly-lit monochromatic background for the purpose of removing the background from the scene and replacing it with a different image or scene.
2. 2
   The visual effect resulting from this technique as well as the colored screen itself, especially blue.
3. 3
   Ellipsis of blue screen of death.