verb: lip-sync; 3rd person present: lip-syncs; gerund or present participle: lip-syncing; past tense: lip-synced; past participle: lip-synced
(of an actor or singer) move the lips silently in synchronization with a recorded soundtrack.
Since about yesterday the "Superbowl Halftime Recorded-Track Extravaganza Blowout" has added "lip-sync" to the description of what happened that day. Now I just heard Steve Gorman (drummer, Black Crowes) on his sports talk radio show joke about RHCP and Bruno Mars lip-syncing at halftime. But since there are a bunch of threads existing on the matter, here is the official "Correct definition of lip-syncing" thread.