Traditionally, mechanics which are not representative of in-game reality. For example, plot points or Drama Deck cards would be examples. Whereas mechanics like falling damage or magic rules are representative of in-game reality, metagame mechanics do not imply anything about the reality portrayed. In Ron Edwards' Big Model, he defines this as "where System and Social Contract meet, without Exploration as the medium."

There is an occasional alternate usage of metagame mechanic, which is an option that is rarely invoked -- i.e. an exception to the "normal" way of resolving a situation.

References:

• Metagame mechanics (The Forge, May 2001)
• Metagame & Mechanics (The Forge, Sep 2003)

I am confused by this definition. How are (for example) Drama Deck cards not representative of in-game reality? If I have a card that gives me a +2 on an attack roll (a typical drama deck card from the Masterbook and Torg decks), and play it, how is that somehow different from the attack roll itself? What about Resource Allocation? games like the Marvel Universe RPG or stat-comparison games like Amber or diceless Fudge? - Anonymous Contributor?

This question gets into the reason for the alternate definition. As noted in the referenced threads, "non-representative" mechanics were historically "tacked-on" to a game system to allow the players to overrule the default resolution system. They were thus "metagame" in both senses.

There are now game designs with resolution mechanics which are entirely "metagame" in the first sense. For many observers, the key distinction is the relationship between Stance and any decisions or evaluations which go into the application of the mechanic. For example, if the mechanic calls for a decision by the player which isn't analogous to a decision made by the character, or if it references a resource or property which doesn't exist inside the game-world, it is a metagame mechanic.

Reference:

• Simulationism: The Right to Dream (Ron Edwards) (See heading "Metagame mechanics")