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.