Lip syncing was the act of simulating a vocal performance by silently mouthing the words, their own or another performer's, generated from a pre-recorded soundtrack. This gave the illusion that the words were sung, or spoken, live, instead of coming from a recording.
Murf, a Mellanoid slime worm, despite being unable to speak, sang a song during a black and white holodeck recreation, prompting Zero to comment that Murf might be good at convincingly lip syncing. (PRO: "Ghost in the Machine")