A tic or twitch was an involuntary movement of one's body.
In 2364, Lore, posing as Data, told Wesley Crusher that he'd been practicing Lore's facial tic. When he asked Crusher if he was doing it right, the boy told "Data" that he should try to forget imitating Lore. Later, after he left, the evil android gave Data a tic and got rid of his.
He used this twitch to "prove" that Data was Lore to William T. Riker, but Data subverted this by signaling with increased twitches. Later, after the Crystalline Entity left, Jean-Luc Picard told a still-twitching Data to get rid of it. (TNG: "Datalore")