special effect

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A visual effect added to a movie or a taped television show during processing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an effect used to produce scenes that cannot be achieved by normal techniques (especially on film)