Metagaming

metagaming, n., v., adv. Also metagamer, to metagame. 1. in RPGs, a form of cheating that involves the use of information gained unfairly, or in a way that makes no sense within the game.
Ex: John's a metagamer -- he attacked Sprinkle's weak point, even though his character couldn't have known about it.
2. in a more general sense, action by a player based on (1) information gained by that player or (2) cooperation between players, which is not reflected in information gained by the PC, or in cooperation between PCs. [GJ1]
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Addendum: In roleplaying, it is vital to separate the world of the gamers from the world of the game, so that PCs will behave in a manner that is consistent with the story. For example, Jeff may know that Todd's character has been replaced by a doppelgänger, but Jeff's character Sir Milas doesn't know it. So Jeff should wholeheartedly play his character as if he has no idea. Metagamers abandon this concept, either because of inexperience (see munchkin) or as a form of cheating. Metagaming and power gaming usually go hand in hand. See to firewall. [GJ1]
Addendum #2: Most games will have some form of metagaming every now and then, especially among the more casual games. Most GMs, save for the most hardline types, usually allow it as long as it doesn't get out of hand, but it is usually when the metagaming is disruptive or blatant cheating when it becomes the type that regular gamers draw the line against.