verb (used with object), verb (used without object)
to synchronize (recorded sound) with lip movements, as of an actor in a film.
to match lip movements with (recorded speech or singing): She did a clumsy job of lip-syncing her big song.
the technical process by which
this is done.
the simultaneous recording of voice and picture, especially the synchronization of lip movements with recorded sound
Also, lip·synch, lip·sync.