sight gag

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

• noun A comic bit or effect that depends on sight rather than words.

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

• noun film, television A visually presented action or device which causes a humorous response without requiring any accompanying commentary or other speech.

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

• noun a joke whose effect is achieved by visual means rather than by speech (as in a movie)