A change in Buffyverse canon, usually made to expand or reinterpret existing storyline history. For example, "You were my Sire, man, you were my Yoda." Spike's sire is explicitly given as Angelus in season two, but later contradicted in Fool For Love, when we see him turned by Drusilla. (Incidentally, Joss has explained this!)
From the Jargon File, version 3.1.0: :retcon: /ret'kon/ [short for `retroactive continuity', from the USENET newsgroup rec.arts.comics] 1. n. The common situation in pulp fiction (esp. comics or soap operas) where a new story `reveals' things about events in previous stories, usually leaving the `facts' the same (thus preserving continuity) while completely changing their interpretation. For example, revealing that a whole season of "Dallas" was a dream was a retcon. 2. vt. To write such a story about a character or fictitious object. "Byrne has retconned Superman's cape so that it is no longer unbreakable." "Marvelman's old adventures were retconned into synthetic dreams." "Swamp Thing was retconned from a transformed person into a sentient vegetable." "Darth Vader was retconned into Luke Skywalker's father in "The Empire Strikes Back"".
Retroactive Continuity. Claiming that something was always true, and that history is now the way it would have been, had it been true. One method of altering something (usually a character) and maintaining consistency.
Retrospective continuity, or reality shift. Agreement between all players and GM to accept as in-game reality, a version of events different from that which was originally played out.
From "retroactive continuity," a term used on r.a.c.m. to refer to a change made to a comic's established continuity in a later comic. This may contradict previous versions of continuity, or it may complement the story and add details without actually overriding anything established. Changing Zoon to Zuun, or Bgztl to Bgtzl, are examples of the former; saying that Cosmic Boy was born on Earth and was a magno- ball champion is an example of the latter.
A change in pre-existing canon.
To retroactively change the continuity of a character or title. Originally, the term "retcon" was used only in cases wherethe interpretation of "facts" from earlier stories is changed,but the facts themselves are preserved. A "patch" was the term used (taken from programmer's jargon) to mean an actual change, rather than merely filling in details. These days, however, "retcon" is used increasingly to mean changes to history as well as to retroactive continuity. So, to "retcon" is to change history, so that something that had existed in the continuity of the fictional universe, not ONLY doesn't exist now, but in the fictional history, NEVER HAS existed. This can be true of an event, of a character, or whatever.
( link / ) Short for retroactive continuity, when previously established game elements (often including the actions of the PCs) are changed after the fact. This can range from "taking back" actions during combat to adding background elements to a character after the game has begun.
Short for retroactive continuity, this is an informal term used to describe historical events in the DC universe that have been added, subtracted or otherwise altered by modern writers. See main article for more comprehensive details.
Retroactive continuity or retcon is the adding of new information to "historical" material, or deliberately changing previously established facts in a work of serial fiction. The change itself is referred to as a "retcon", and the act of writing and publishing a retcon is called "retconning". Retconning can be done either on-purpose, or accidentally, wherein a break in continuity is not noticed until later and is then 'blessed' by later writers or editors.