

The decks in a metagame will never be perfectly balanced. Metagames are like living puzzles that try to solve themselves. Then, people notice that shatter has become uncommon, so they begin to play artifacts again.

This is another example of metagaming because the players have changed their decisions in anticipation of their opponent's decisions. Then, people notice that nobody is playing artifacts so they take Shatter out of their decks since it is worthless without artifacts to target. These people are metagaming because they have changed their decisions in anticipation of people playing Shatter. One example from Magic is the card Shatter, which says " destroy target artifact." Shatter is very powerful, and so to avoid it most people have stopped playing artifacts. The term “metagame” is also used to describe the game around the game, including the Organized Play system, online resources, a library of material, and numerous communities. The practice of tuning a deck or adding sideboard cards in order to have a better chance to defeat the most popular decks is called metagaming.

Simply put: "what everyone else is playing." The decisions the players make in what decks they play and what cards they put into those decks in acknowledgement of their predictions about what potential opponents might choose themselves is what is called 'playing the metagame'. In Magic, "Metagame" commonly refers to the popularity of decks, and sometimes specific cards.
