Retcon is a contraction of the term "retroactive continuity". It describes the act of changing previously-established details of a fictional setting, often without providing an explanation within the context of the setting for the changes.
The term was coined by comic book writer Roy Thomas in his 1980s comic book series All-Star Squadron which concerned the DC Comics superheroes of the 1940s. The earliest known use of the term is from Thomas' letter column in All-Star Squadron #20 (April 1983). The term was shorted to "retcon" in the late 1980s on USENET.
Retconning is common within the fictional universes of comic books, especially those of large shared comic book houses such as Marvel Comics and DC Comics, due to the lengthy history of publishing and the large number of independent authors contributing to their development. Retconning also occurs in television shows, radio series, and series of books.
Some forms of retconning do not directly contradict previously-established facts, but instead retroactively "fill in" missing background details necessary for current plot points.
Related to this is the concept of shadow history or secret history, in which the events of a story occur within the bounds of already-established (especially real-world historical) events, but have been hitherto unrevealed.
Retroactive continuity is similar to, but not exactly the same as, plot inconsistencies introduced inadvertently; retconning is usually done deliberately. Unpopular retcons are often unofficially combatted through the judicious use of Krypto-Revisionism; this is the phenomenon of a fan base deciding to ignore a particular retcon.
While retconning was usually done in the past without explanation, DC Comics has on rare occasions promoted special events dedicated to retroactively rewriting the history of the DC Comics universe. The most important and well known such event was the mini-series Crisis on Infinite Earths; this was a profound change that allowed for wholesale revisions of their characters. A second major retcon in DC Comics was in a similar event called Zero Hour.
Retconning has occurred a number of times in the history of the Star Trek universe. The various Star Trek television series and movies were produced over many decades, with multiple writers and producers. In both cases significant amounts of time, effort, pages and film have been used by later writers to explain or qualify apparent inconsistencies from previous stories.
When Star Trek: The Motion Picture was first released, Gene Roddenberry and others associated with the franchise claimed that the radically different visual appearance given the Klingons in the film was how they were always supposed to have looked; they simply did not have the make-up budget to do this on the original 1960s television series. As such, viewers were asked to overlook this difference, and act as if Klingons had always had forehead ridges. This is an example of strong retconning. However, an episode of Star Trek: Deep Space Nine, produced in the 1990s, explicitly addresses the fact that Klingons in the 22rd century did not have forehead ridges; this fact was acknowledged within the continuity of the show itself. Commander Worf, a Klingon, addresses the fact, admitting within the context of the show that at least some Klingons did at one time not possess forehead ridges. When asked why, Worf showed significant embarrassment, and simply stated "It is not something we like to talk about." While the scene was played for humourous effect, the show clearly introduced a new retcon that effectively tied together previous and current show continuity — At least some Klingons did not have forehead ridges, for a reason that modern Klingons now consider embarrassing. (Many fan theories have been proposed.)
A plot thread in the fifth season of the television series Buffy the Vampire Slayer played with the idea of the retcon. At the end of the first episode of the season, Buffy meets a girl who she recognises (and, in subsequent episodes, everyone else also recognises) as her younger sister Dawn — despite the established fact, reiterated earlier in the same episode, that Buffy is an only child. It is eventually explained that Dawn's arrival was accompanied by a magic spell that changed people's memories and altered reality to provide apparent evidence (family photos, etc.) that Dawn had been around for years. The result is something that looks a lot like a retcon but isn't actually, because there is no change in the established facts of the series (ie. it remained an established fact that Buffy had not had a sister in the previous four-seasons-and-most-of-one-episode).
Inconsistencies often come up in the Star Wars universe, usually between the Expanded Universe and the films.Examples of retconning
Sometimes writers introduce dialogue and plotlines that explicitly address, or allude to, the discrepancies, so that changes can be explained away within the current continuity of the series. For example:
