Retcon: Abbreviation for “retroactive continuity,” “reframing past events to serve a current plot need.” (Source: TV Tropes.) Also used as a verb meaning “to revise history.”
“Retroactive continuity” first appeared in print in a scholarly 1973 book, The Theology of Wolfhart Pannenberg, where it was an English translation of a German phrase, “Kontinuät nach rückwärts.” A Wikipedia entry quotes the relevant passage:
Pannenberg’s conception of retroactive continuity ultimately means that history flows fundamentally from the future into the past, that the future is not basically a product of the past.
The term was popularized in the 1980s by the comic-book and science-fiction communities; Damian Cugley has claimed credit for shortening it to “retcon” in a 1988 Usenet discussion of the comic book Saga of the Swamp Thing. It has since been applied to many kinds of serial fiction, including television shows such as “The Simpsons,” “in which the timeline of the family's history must be continually shifted forward to explain them not getting any older” (Wikipedia).
In the last decade or so, “retcon” has been appearing in political journalism as well, substituting for the previously popular “revisionism” and its derivatives.
My husband on Obama's unconvincing attempt to retcon "If you like your insurance, you can keep it. Period." http://t.co/qQJRkv4x92
— Megan McArdle (@asymmetricinfo) November 5, 2013
The link goes to an article by Peter Sunderman in Reason:
If you’re a fan of comic books or other types of serial fiction, you’re probably familiar with the concept of the “retcon”—a made-up word that stands for “retroactive continuity.” …
I wonder if President Obama is a comic book fan. Because with the updated version of his oft-repeated promise that individuals who like their health plans can keep them, he’s essentially retconned himself.
I found citations for “retcon” from the George W. Bush era as well. An October 2004 post in the blog Burnt Electrons was titled “So Now We Retcon Wars...” A satirical May 2005 post on the Defective Yeti blog invented a quote for Ken Mehlman, then chairman of the Republican National Committee: “Americans have always been very accepting of our retcons.”
From TV Tropes:
As the number of twists and misdirections in a story becomes higher, it becomes more difficult to tell whether an event actually is a retcon (which implies that the writers changed their minds), or a misdirection (which implies that the writers intended the “retconned” version all along, and had been deliberately misleading the audience before). In some cases, it is impossible to tell, short of reading the author's mind (even then, it might not helped, as it's entirely possible for an author to be on the fence about what they're planning to do).