A universe parallel to that of the regular *Star Trek* franchise, the Mirror Universe features the same characters as the so-called Prime universe, but with the democratic Federation replaced by the corrupt and authoritarian Terran Empire and the main characters in different stations as a result. Later in its history, the Empire is overthrown by the Klingon-Cardassian Alliance

The mirror universe was so named because many people and places seemed to be opposites of their characteristics in the prime universe, with numerous good aspects now evil and vice versa, thus "mirror"-like.
See the "Mirror Mirror" episode from TOS.

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