The Mirror Universe

The "mirror universe" is an informal name for a parallel universe first recorded as visited by James T. Kirk and several officers from the USS Enterprise in 2267. This parallel universe coexists with the "prime" universe in the same space, but on another dimensional plane. The mirror universe was so named because most places, ships, and people who existed in the regular universe also existed there, although often as the antithesis of their characteristics in the prime universe, with numerous good aspects and evil aspects and vice versa, thus "mirror"-like.