:McGurk effect:

Pronunciation GB: məˈɡɜːk, GA: məˈɡɜrk

A perceptual effect that demonstrates that visual cues may influence the perception of speech sounds. A video is shown of a speaker producing an utterance, but the sound of this is replaced by a recording of a different utterance by the same speaker. For example, the picture may show the speaker producing the sequence ɑɡɑ, while the sound-track is of the sequence ɑbɑ. This results in the perception of the sequence as ɑdɑ when the listener is watching the picture, but as ɑbɑ when she is not.

A video demonstration in a new window/tab.

McGurk H. & MacDonald J. (1976) Hearing lips and seeing voices. Nature, 264:746-748