|See also:||Alternate Universe, Doppelganger|
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A mirror universe is a type of alternate universe in which familiar characters are given reversed characteristics. Heroes might be evil, or at least far more ruthless and aggressive. This is often indicated visually by more sexually revealing clothes, longer hair and/or facial hair, or quasi-fascist imagery.
The term originates from Star Trek, which has several episodes portraying a canonical alternate universe in which the peaceful Federation never existed, replaced by the brutal, conquering Terran Empire. In terms of air dates, the first "Mirror Universe" episode was the TOS episode Mirror, Mirror.  
In general when a fanwork is stated to be set in a 'mirror universe,' especially Star Trek's mirror universe, it means the story will be darkfic. The canonical "moral inversion" of the Mirror Universe setting gives the fanwork creator the opportunity to depict dark subjects such as torture, death, rape, dubiously consensual sex, or fantastical/unsafe BDSM practices. (In "Mirror Mirror" it was canonically established that Enterprise crew members can use sex as a means of advancement through the ranks.) Characters from the regular universe and the Mirror Universe may also interact, giving the writer an opportunity to explore doppelganger or Evil Twin scenarios.
However, a "mirror universe" fanfic can sometimes also refer to a more general type of reversal, such as a Stargate SG-1 fic where Jack O'Neill is a scientist and Daniel Jackson is a military officer.
Star Trek mirrorverse
- issenterprise, a community for "Fannish media focused on the mirror universe of Star Trek." Created on September 05, 2009, with 346 posts, 4,159 comments and over 360 members as of October 2010.
- SakuraMiko's mirror_universe Bookmarks on Delicious
- Spock/McCoy Mirror-Fic Master List by yami-no-kaiba
- The Crack in the Mirror (1994)
- Federation and Empire
- One Way Mirror (1980)
- The Renegades (Star Trek: TOS zine by Sarah E. Thompson & Linda K. Rossi) (1982)
- Revolution (1995) (Led Zeppelin xover)
- Victim of the Nova (1982)
- Visions of Empire (1989)
- Within the Mirror (1988-2006), K/S zine series dedicated to the Mirroruniverse
Sample Star Trek Mirrorverse Fic
Gallery of Mirror Universe Zine Covers
On March 25, 2010 a post was made to the LJ community ontd_startrek announcing "ontd_startrek IS NOW THE PROPERTY OF THE TERRAN EMPIRE. (ALL HAIL THE GLORIOUS EMPIRE! BOW DOWN BEFORE THE MIGHT OF THE EMPIRE!)"  At first, not realizing that the new "overlords" named in the post were actually the ontd_startrek mods with slightly different names, some community members suspected an actual invasion by trolls and reported the "takeover" to LJ Abuse.
The confusion was quickly cleared up, and "Mirrorverse Week" was soon underway, celebrating everything Mirrorverse.  In celebration, all posts that week were Mirrorverse-themed, and community members added goatees/facial hair to their icons, two hundred and thirty-eight of which were collected in one giant macro. On April 1 the community returned to normal, although there were ominous hints that at some point in the future, the "evil mods" might return.
Other Canon Mirror Universes
- Star Trek: DS9 also featured a number of mirror universe episodes
- Hercules: The Legendary Journeys and Xena: Warrior Princess featured a "mirror universe" where Hercules is randomly evil (and has a goatee) and Ares, god of war, is a pacifist (and lacks his usual goatee).
- The Doctor Who episode "Inferno" sends the Third Doctor to an alternate universe where Liz Shaw and the Brigadier are evil. The evil!Brigadier has an eyepatch and scar, but lacks his usual mustache.
- Red Dwarf had an episode where they went to a genderswapped alternate universe. There was also "Dimension Jump" which introduced a Rimmer from an alternate universe; in contrast to canon's weaselly Arnold Rimmer, the alternate "Ace" Rimmer was charming, heroic and sexually confident.
- The DC Comics parallel universe Earth-3, in which the "Jokester" is a hero and Thomas Wayne is "Owlman," a sadistic villain, and most other characters are similarly warped or flipped.
- Charlie Jade's Alphaverse shows characteristics of a mirror verse.
- Fringe's overall arc revolves around the interactions of characters from the primary universe with at least one parallel universe where things haven't gone so well.
- The Middleman ended with a mirror universe episode that implied/revealed surprise information about some characters' moral alignment in the normal universe.
Sample Non-Star Trek Mirrorverse Fanworks
- Starsky & Hutch: A World Full of Jokers by Verlaine. Starsky is sent to a mirror verse Bay City in which he and Hutch are corrupt cops.
- Stargate Atlantis: Crimes Against Humanity by seperis (2006-2008). Mirror verse where most of SGA's main characters are psychopaths imprisoned on Atlantis, which is used as that universe's Alcatraz.
- Stargate Atlantis: Make the Devil Feel Surprise and sequel Giving the Devil His Due by telesilla (2009-2010). Mirror verse where to curry favor, his sister hands 16-year-old Rodney off to the sadistic Colonel Sheppard.
- Merlin inverted or "mirror universe": Seven Magpies by syllic (2010), in which Merlin is a prince and Arthur his servant
- Stargate Atlantis inverted or "mirror universe": Teamwork by Magus Minor and Methaya (2005). They're still on Atlantis, but Rodney McKay is with the military and John Sheppard is a scientist.
- Beard of Evil at TV Tropes