To lip-synch is to move your mouth along with a recording, so that it looks like you are singing or speaking the words, when really the sound is coming from the recording. You see it everywhere: singers on TV shows where the live sound would be bad, actors in a music video, a dubbed film where the mouth has to match another language. It is a verb ('she lip-synched the whole song') and also a noun-like word in phrases such as 'a lip-synch performance' or 'lip-synch to a song'. It is written several ways — lip-synch, lip-sync, lipsync — and 'lip-sync' is now the more common spelling; the '-synch' spelling is a bit older. It comes from 'lip synchronization', so the noun 'lip-synch' can also just mean the matching itself: in film work people say 'the lip-synch is off' when the sound and the mouth do not line up. In everyday talk it often carries a small judgement — if you say a singer was lip-synching at a concert, you usually mean the audience was a bit cheated. But not always. Drag performers lip-synch on purpose as an art form, and nobody is pretending otherwise; there, it is the skill of acting a song with your face and body. And people lip-synch alone in the car or on TikTok just for fun, with no pretending involved at all.