|See also:||Fanon, Canon, Jossed, Prequel|
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To retcon something is to replace a previously established canon fact with different canon. The term is a portmanteau of retroactive continuity and originated in comics fandom. When fanfic is written in order to correct, undo or explain away unwanted canon, it may be called Fix-it fic or denialfic (such as the multitude of SND stories in the Beauty and the Beast fandom) but only TPTB can officially retcon canon.
Western comics fandoms are notorious for repeatedly piling retcons on top of retcons until even the characters grow confused about the facts of their existence. Power Girl is one character who at one point, canonically, had no idea whether she was a human, Atlantean or Kryptonian, or where her powers come from.
For instance, comics writers who feel that Doctor Doom was written OOC in the past have often revealed that any out of character behavior was actually performed by a robot (or "Doombot"), not the real Doctor Doom.
Perhaps one of the more famous Marvel comics retcons was John Byrne's resurrection of Jean Grey. In a previous storyline, Jean had blown up the sun of a populated solar system, then died on the moon. Byrne's retcon stated that Jean had in fact been replaced by the Phoenix Force before that storyline, so that she was not only alive again, but had never committed any unforgivable acts.
Another Marvel retcon is the back-story of Jessica Jones which includes attending school with Peter Parker, past membership of The Avengers as the minor superheroine Jewel, and participation in many canon events, leading to her retiring as a superhero and becoming a private detective.
A DC universe retcon popular with Smallville fans and Clark/Lex shippers revealed that Kon-El, previously thought to be a clone of Clark Kent with a slight amount of human DNA, was actually a clone of both Clark Kent and Lex Luthor.
On Buffy the Vampire Slayer, the character Spike specified in season 2 that Angel was his sire, i.e. the vampire who made him a vampire. This is contradicted in a flashback in season 5, which shows that Drusilla is Spike's sire. One fact merely supercedes the other, without explanation; this is understood as a retcon rather than a continuity error. Given her unusual status, all memories of Dawn Summers' life prior to Season 5 are retcons.
Joss Whedon is a longtime reader and writer of comics, and appears to have no difficulty with superceding his own canon when a better idea comes along. He has done this often enough that it has led to the term "being Jossed" for when retcons or new canon contradict fanon or fanworks.
Most episodes of Arrow include flashbacks to Oliver Queen's missing years which often retcon previous canon. The 2014 Flash series also retcons occasionally, but has time travel to explain the changes.
On The X-Files, the My Struggle four parter of season 10 and season 11 episode retconned a large part of the shows mytharc that had been established in the first nine season where a cabal of shadow government men centered around the Cigarette-Smoking Man worked together with alien colonists to prepare a hostile takeover of planet earth. The date for the beginning of this invasion had been the end of the Mayan calendar on December 22nd 2012. In the episode My Struggle III it's stated that the invasion was dropped because the colonists lost interest on the planet because humankind destroyed earth via pollution, draining of natural resources and global warming and made in unappealing to the aliens.
The TV series Torchwood features a drug called retcon, which erases a person's memories. The main characters, members of an underground, clandestine organization, use the drug to "retcon" themselves out of the memories of people who have observed more than they should.