The Jargon File (version 4.4.7, 29 Dec 2003):

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.?

    [This term 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]

    [1993 update: some comics fans on the net now claim that retcon was
    independently in use in comics fandom before rec.arts.comics, and have
    citations from around 1981. In lexicography, nothing is ever simple. ?ESR]


The Free On-line Dictionary of Computing (30 December 2018):

retcon

   /ret'kon/ retroactive continuity.

   The common situation in fiction 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.

   This term was once thought to have originated on the Usenet
   newsgroup news:rec.arts.comics but is now believed to have
   been used earlier in comic fandom.

   [Jargon File]

   (1994-12-08)