In role-playing games, meta-gaming is an "out of character" action where a player's character makes use of knowledge that the player is aware of but that the character is not meant to be aware of. Meta-gaming while taking part in relatively competitive games, or those with a more serious tone, is typically not well received, because a character played by a meta-gamer does not act in a way that reflects the character's in-game experiences and back-story.
Examples of meta-gaming include:
Adjusting a character's actions based on foreknowledge of the long-term intentions of the gamemaster.
Gaining knowledge from Out-Of Character.
Using knowledge from a previously played or dead character.
Using certain types of attack or defense based on the strengths and weaknesses of an opponent that the player's character is unaware of.
Acting on any knowledge that the character is not aware of (such as creating gunpowder in a Dark Ages or Middle Ages setting).
Adjusting a character's behavior towards other player characters based on real-life relationships with other players.
Using knowledge of the game's mechanics to gain an advantage in the game by having the character do something incompatible with that character's personality.
Deciding on a character's course of action based on how the game's mechanics will affect the outcome without more significant regard placed on how the character would actually behave.
Any action that is based upon the knowledge that one is playing a game.