Metagame

Metagame (Greek: μέτα = “about”, “beyond”), literally "a game outside the game," is a prediction of how others will make decisions in a game based on their personality or their previous decisions. A metagame can exist in any game in which the opposition is human or portrays some sort of artificial intelligence and the competitors make choices.

The Magic metagame
In Magic, "Metagame" commonly refers to the popularity of decks, and sometimes specific cards. 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'. 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. 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.

Example
Metagaming is often cyclical. 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. These people are metagaming because they have changed their decisions in anticipation of people playing Shatter. Then, people notice that nobody is playing artifacts so they take Shatter out of their decks since it is worthless without artifacts to target. This is another example of metagaming because the players have changed their decisions in anticipation of their opponent's decisions. Then, people notice that shatter has become uncommon, so they begin to play artifacts again. This, yet again, is metagaming. This cycle can continue indefinitely.

In Standard
Metagames are like living puzzles that try to solve themselves. The decks in a metagame will never be perfectly balanced. Within the first week of a set's release, the player-base as a whole will have put far more collective hours into finding the best deck for Standard than R&D can during the entire development period. The strategy the developers have for creating metagames that don't solve themselves within the first few weeks is making cards that are, as a whole, well balanced, but also do enough different powerful things that all of the decks in the metagame have room to shift as time moves on. One example of this is Oko, Thief of Crowns. This is an example because since so many Simic decks are running it, it has become very expensive, and, as a result, more decks are running copies of Noxious Grasp and Veil of Summer.