Wiktionary
sight gag
n. (context film television stage performance English) A visually presented action or device which causes a humorous response without requiring any accompanying commentary or other speech.
WordNet
sight gag
n. a joke whose effect is achieved by visual means rather than by speech (as in a movie) [syn: visual joke]
Usage examples of "sight gag".
The being in question who wandered in a Murfle (we've seen them before, but a more in-depth description is needed for this sight gag to work).
Randy's head protruding from a suit is a sight gag that will be good for grins at least through lunchtime.