This longish discussion of retcons in comics is quite a read.
A retcon, for those of you too lazy to follow the link, is a device used in long-form narratives to RETroactively change the CONtinuity of the story. Because comics are among the longest narratives we have, it comes up a lot in that world.
For nongeeks, perhaps the most widely known retcon came at the end of Dallas‘s 1985-86 season, when Pam found Bobby in the shower — which rendered the entire season a terrible, terrible dream. A more drastic retcon just happened to James Bond, as Casino Royale is the story of an operative only just promoted to 00-status; for Daniel Craig’s Bond, there has been no SMERSH or SPECTRE, no Cold War, no undersea madmen, no Goldfinger, no marriage to Diana Rigg, and, sadly, no Octopussy. (Bond’s retcon is a drastic enough example to warrant being called a reboot, but that’s just a special type of retcon.) Comic retcons are typically (but not always) less ham-handed, but it’s this sort of shift.
Comics, unlike soaps, are faced with an undeniable need for retcons, precisely because they’re long-running but also, at least to a point, at least somewhat frozen in amber [1]. If they didn’t keep shifting or handwaving about some aspects of Superman’s backstory, for example, we’d be dealing with a decidedly geriatric Man of Steel; at his introduction in 1938, he was supposedly in his late 20s, which puts his birth at about 1910[2]. I’m pretty sure the current comic incarnation isn’t supposed to be 98 years old. Even Spiderman would be at least middle aged if his story moved in real time; he was a student in 1962.
To add to the complexity, comic publishers are forever having crossover events where Batman and Superman team up to fight Evil Dude X, which means their continuities are blended; a retcon for one will necessarily affect the other. In the persistent world of the DC (or Marvel) universe, this gets very complicated very quickly.
Anyway, that’s a retcon. Check out the first link for discussions of them done well (Frank Miller’s work on Daredevil; they actually skip my favorite) and badly (pretty much the last 20 years of the Legion of Super Heros), all through the lens of the gawdawful bullshit they’re pulling with Spiderman these days. (There’s a literal deal with the Devil that undoes pretty much the last 2 or 3 decades. I shit you not.).
(Via Wil.)
Notes:
1. An amusing example of retconning is ongoing with the Simpsons, who have taken being frozen in time very seriously. The show’s nearly twenty, so flashbacks to teen/early 20s time periods for Marge and Homer have ratcheted from the 70s through the 80s and into the 90s.
2. The cinematic Supes has also been retconned, and recently: Superman Returns is itself a bit of a retcon in that it branches off the film continuity after Superman II, and renders moot all subsequent Reeve films.)