A card that inadvertently turns face up when it isn't supposed to, such as during the deal in a draw game.
A card that is turned face up by accident.
A card whose face has been deliberately or accidentally revealed to players normally not entitled to that information during the play of the game. Various games have different rules about how to handle this irregularity.
A face-up card, or upcard, in Stud and in community cards resting in the center of the table.