When you pretend you are singing by synchronizing your lip movements to a recording, you lip-synch—the vocal equivalent of playing “air guitar.” Some people mistakenly think the expression is “lip-sing,” and they often omit the required hyphen as well. Note that you can lip-synch to speech as well as singing.

Many writers use the spelling “sync” rather than “synch.” Users of each form tend to regard the other as weird, but in contemporary writing “sync” clearly prevails.