A Mirror Match is a game played against an opponent who has a nearly identical deck. The name derives from the fact that both players will go through similar motions and actions, with similar cards being played in the same sequence. Thus it would evoke a feeling like playing in front of a mirror.
Mirror matches are particularly common in tournaments, as decks with a perceived large probability of success are adopted by a large amount of players. In such cases, tuning the deck or adding specific sideboard cards which provide a competitive edge in the mirror match becomes part of the Metagame.