A lip dub is a type of video that combines lip synching and audio dubbing to make a music video. It is made by filming individuals or a group of people lip synching while listening to a song or any recorded audio then dubbing over it in post editing with the original audio of the song. [...] Often, they look like simple music videos, although many involve a lot of preparation and are well produced.

http://en.wikipedia.org/wiki/Lip_dub

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