(shortened form of RETroactive CONtinuity; first made popular in the comic book world)
1. (original meaning) Adding information to the back story of a fictional character or world, without invalidating that which had gone before.
2. (more common usage) Adding or altering information regarding the back story of a fictional character or world, regardless of whether the change contradicts what was said before.
1. Although they had previously been shown to have two other sets of parents, the retcon of making Quicksilver and the Scarlet Witch the children of Magneto only altered the meaning of past events, not what had happened.
2. Retconning Dawn Summers into "Buffy the Vampire Slayer" in the fifth season was one of the rare instances where the fact that history has been altered for our characters was recognized in the story, even though the characters all still remembered the "new" versions of events.