The Mirror World was a fourth dimension of the universe containing Earth. One could be transported there by being reflected through several mirrors positioned at particular angles from one another.
The Mirror World was home to a city inhabited by Orinocas, a race of telepathic warlike women.
The existence of the Mirror World was discovered by the first Mirror Master, Sam Scudder. At first hesitant to travel there due to the possible danger, he eventually did as a means of escaping prison. However, he had no way to return home, and eventually returned by luring the Flash there. The speedster vibrated them both back to Earth.