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Metagaming is a broad term usually used to define 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.
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.  
Examples of metagaming
- There is a special set of moves in chess which allows a player to win in four moves. Competitor A has been watching Competitor B play chess, and the past five games in a row Competitor B has attempted to use this four-move win. When Competitor A sits down to play against Competitor B, Competitor A will be metagaming if he/she plays in a way that will easily thwart the four-move checkmate before Competitor B makes it obvious that this is what he/she is doing.
- In role-playing games, a player is metagaming when they use knowledge that is not available to their character in order to change the way they play their character (usually to give them an advantage within the game), such as knowledge of the mathematical nature of character statistics, or the statistics of a creature that the player is familiar with but the character has never encountered. In general, it refers to any gaps between player knowledge and character knowledge which the player acts upon.
- Computer game development projects, in which the game company seeking to develop new and better games will set its developers to propose and test alternative designs and rules among themselves, until they arrive at a new design that they can expect will be popular. This development process is itself a kind of metagame.
- In the popular trading card game Magic: The Gathering players compete with decks they have created in advance and the "metagame" consists of the deck types that are currently popular and expected to show up in large numbers in a tournament. The knowledge of metagame trends can give the players an edge against other participants, while playing (quickly recognizing what kind of deck opponents have to guess their likely cards and moves) and more importantly in the deck building process, by selecting and adapting designs to do well against the popular deck types at the expense of performance against rarer ones. It's also possible to bluff opponents into expecting cards that aren't there, or to surprise the competition with novel decks that nobody is prepared for. The secondary market of cards is heavily influenced by metagame trends: cards become more valuable when they are popular, often to the point of scarcity.
- In fighting games such as Super Smash Bros. Melee, metagaming may occur at the character select screen. The opposing character has various strengths that can be avoided and weaknesses that can be exploited more easily depending on the character you choose provided you are aware of those strengths and weaknesses (called a "match up"). For a basic example, a character with a projectile attack has the advantage over a grappler who must be close to the opponent to be effective. Match up metagaming is very important in tournament settings. In recent fighting games, blind select has been implemented for online modes. This makes it so that neither player can see what character the other player chose. In tournaments, players have the option to opt for a blind select where they tell a judge in confidence the character they intend to select in the match, making their character choice mandatory.
- Many logic puzzles allow an analogue of metagaming. By convention, logic puzzles are only considered well-constructed if they have a unique solution. When solving a puzzle, one might notice that if a certain candidate symbol were placed in one square, there would be multiple ways to complete another part of the puzzle, and no extra information could possibly decide between them. Ruling out that candidate on these grounds would be metagaming.
Within actual entertainment games, the term metagame is used to describe either a game system layered over the game system, to increase enjoyable complexity, or a game system by which game rules are created, such as Nomic. Nomic is a sophisticated and simple example of a metagame popular as a pastime among philosophers and mathematicians, but has also spread widely among other people as a recreational option. Some card games and board games allow dynamic rule changes depending on extraneous events, such as distinct states of weather or commercials on the television.
Adaptation to a specific gaming environment
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. Another example would be in the multiplayer online battle arena genre or Warcraft III: The Frozen Throne, where the metagame refers to the current playstyle and hero choices being favored by high level teams in recent competitive play. In EVE Online, the metagame refers to the styles of fitting, engagement tactics and fleet doctrines used by the playerbase on small and large scales, where knowledge of the types of NPC and Player encounters that are likely to be found influences the fleet composition directly. Recent changes in EVE's Odyssey patch directly address the metagame to balance many of the ships, bringing up in power or use those which were less favoured while toning down those that dominated universally, such as the old "Hurricane" battlecruiser.
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 a character act on knowledge that only the player has access to (such as tricking a Medusa to stare at a mirror when the character has never even heard of Medusa and should 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 GM's tendency to have (or not to have) mercy on players whose characters do things that would cause them to fail at their objectives.
- Paradoxes of Rationality: Games, Metagames, and Political Behavior - Nigel Howard 
-  Rationality: Games, Metagames, and Political Behavior - Nigel Howard.
- "EVE Online: Odyssey Ship Balancing". Archived from the original