(video games, countable) An item that grants the player an extralife in the game, staving off game over by one more try.
(video games, uncountable) In a game with multiple players, an indicator or identifier of player 1.
1982, Harlington Wood, Jr., Atari v. North American Philips Consumer Electronics Corp., 672 F.2d 607, n. 2 (7th Cir. 1982):Before any player registers points, the PAC-MAN scoring table displays in white "1 UP" and "2 UP" where the scores will appear.