special effect

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A visual effect added to a movie or a taped television show during processing. Often used in the plural.

Wiktionary

1. n. film, television An effect that cannot be reasonably achieved by normal means, as for example by the digital manipulation of previously filmed footage.

WordNet 3.0

1. n. an effect used to produce scenes that cannot be achieved by normal techniques (especially on film)