['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. E.g., 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."
[This is included because it is a good example of hackish linguistic innovation in a field completely unrelated to computers. The word 'retcon' will probably spread through comics fandom and lose its association with hackerdom within a couple of years; for the record, it started here. -- ESR]