The mirror match is one of the most common concepts seen in the vast majority of fighting games, in which two players may pick the same character on the character select screen. In the single-player mode, the player may also face off against a CPU double of their selected character.
Although virtually no modern fighting game is released without this functionality, this was not always the case. The original incarnation of Street Fighter II and other fighting games of its era blocked players from selecting identical characters, with the exception of cheat codes or devices that would trick the game into allowing such functionality. Notably, the Street Fighter series first began to allow mirror matches with the release of Street Fighter II: Champion Edition, making it possible for both players to pick the same character with different colored sprites.
The origin of the phrase "mirror match" can be traced to the original Mortal Kombat. In the game's single-player arcade mode, the player must eventually face a CPU-controlled double of their chosen character. This match is referred to as the "Mirror Match" on the game's tournament ladder.