Metagaming is any strategy, action or method used in a game which transcends a prescribed ruleset, uses external factors to affect the game, or goes beyond the supposed limits or environment set by the game. Another definition refers to the game universe outside of the game itself. Metagaming differs from strategy in that metagaming is making decisions based upon out-of-game knowledge, whereas strategies are decisions made based upon in-game actions and knowledge.
In simple terms, it is the use of out-of-game information or resources to affect one's in-game decisions.
The term metagame is a mathematic descriptor for set interaction governing subset interaction. The term passed from military use into political parlance to describe events outside conventional bounds that, in fact, play an important role in a game's outcome. For example, a military operation might be a game with its political ramifications being the metagame. Similarly, the passage of a law might be a game, with the political environment into which that law fits being the metagame.
Similarly, a game might be the passing of a law lacking majority support. The group opposing the law, benefiting in the metagame from the passage of said law, encourage their own members to vote in favor of the legislation.
Metagaming might also refer to a game which functions to create or modify the rules of a sub-game with the purpose of maximizing the subgame's ruleset. Thus, we might play a metagame of optimizing the rules of "chess-like" games to maximize the satisfaction of play, and perhaps arrive at the rules of standard chess as an optimum. This is related to mechanism design theory in which the metagame would be to create or make changes in the management rules or policy of an organization to maximize its effectiveness or profitability. Constitutional design can be seen as a metagame of assembling the provisions of a written constitution to optimize a balance of values such as justice, liberty, and security, with the constitution being the rules of the game of government that would result.
Another game-related use of Metagaming refers to operating on knowledge of the current strategic trends within a game. This usage is common in games that have large, organized play systems or tournament circuits and which feature customized decks of cards, sets of miniatures or other playing pieces for each player. Some examples of this kind of environment are tournament scenes for card games like Magic: The Gathering, or tabletop war-gaming such as Warhammer 40,000 or Flames of War.
Such metagaming could include compiling lists of what race or army choices are being used in a specific region or tournament scene, and tailoring your own army to fight the majority units, for example, knowing that Space Marine variant armies are the largest group of potential opponents, and modifying your own army with equipment which counters the strength of that majority force, or preys upon that majority group's weakness. By doing so, the player is metagaming, as they are attempting to improve their chances for victory by using information outside what will actually take place in a match.
Recently the term metagame has come to be used by PC Gaming shoutcasters to describe an emergent methodology that is a subset of the basic strategy necessary to play the game at a high level. The definitions of this term are varied but can include "pre-game" theory, behavior prediction, or "ad hoc strategy" depending on the game being played. An example of this would be in StarCraft where a player's previous matches with the same opponent have given them insight into that player's playstyle and may cause them to make certain decisions which would otherwise seem inferior.
In role-playing games, metagaming is a term often used to describe players' use of assumed characteristics of the game. In particular, metagaming often refers to having an in-game character act on knowledge that only the player has access to (such as tricking Medusa to stare at a mirror when the character has never heard of Medusa and would not be aware of her petrifying stare). For instance, a player might adjust his character's actions if the player has some foreknowledge of the long-term intentions of the gamemaster, or, more commonly, the gamemaster's tendency to have (or lack) mercy on players whose characters do things that would cause them to fail at their objectives. A player changing how they play the game based on their knowledge of the gamemaster would be metagaming.