1. 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.
EX. Meta Knight, legendarily, was a metagame breaking character in Super Smash Bros. Brawl that, in order to keep the competitive scene interesting, was often banned from tournaments. Other examples of metagame can be found in everything from messing with the knowledge/skillset of your opponent in chess to gaming the metagame of your local Friday Night Magic by playing against the preferences of the usuals by building a deck that counters their favored archetypes.
Honestly, understanding and gaming the metagame is often the most important part of competitive sports and games these days.