retcon
A fictional setting in which a new storyline explains or changes a previous event or attaches a new significance to it.
TLC: What do you say to the Legion fans who have become disenfranchised with the series due to its various retcons and reboots?
Where it comes from
A blend of retroactive + continuity. The term retroactive continuity was popularized by comics writer Roy Thomas, who was known for writing superhero comic books set decades in the past, such as All-Star Squadron, and attributes it to an anonymous source.
Adapted from the Wiktionary entry retcon (CC BY-SA 4.0).