Retcon - Wikipedia

Retcon is a contraction of the term "retroactive continuity." It describes the act of changing previously-established details of a fictional setting without providing an explanation within the context of the setting for the changes.

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. While it was usually done in the past without explanation; now DC Comics often stages a cataclysmic event such as depicted in the mini-series, Crisis on Infinite Earths that profoundly changes the setting to allow for wholesale revisions of their characters. Retconning can be found in other media as well; a prime example is the Star Trek franchise, which has been produced over many decades and 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.

Another form of retconning does not directly contradict previously-established facts, but instead retroactively "fills in" missing background details necessary for current plot points.

Retroactive continuity is similar to but not exactly the same as plot inconsistencies introduced inadvertently; retconning is usually done deliberately. Retcons may be combatted through the judicious use of Krypto-Revisionism.

Examples of retconning:

Erasing an entire season of the soap opera Dallas, as one character's dream.
Frasier Crane's father being a living ex-cop on Frasier instead of a dead research scientist as stated in Cheers.
Differences in the appearance of Klingons between the original Star Trek series and the movies and later series.
Spike having blond hair in some flashbacks in Buffy the Vampire Slayer, and brown hair in other flashbacks set during the same time.

Note that the first three are arguably not true retcons, as there are (however flimsy) explanations for these discrepancies in the fictional worlds.

Notable instances of things that look like retconning but aren't:

In the first episode of the fifth season of Buffy the Vampire Slayer, Buffy mysteriously acquired a younger sister who all the characters treated as if she had always been there. This turned out to be due to a magic spell that affected the characters' memories, not to a 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).