Retcon is short for "retroactive continuity". A staple of long-running comics, it refers to any fact that is contradicted or altered by later material. An instance of this is known as a "retcon", and it can also be used as a verb ("they totally retconned that, she's actually still alive").
As mentioned way back in Episode 001 - The Strangest Podcast of Them All, retconning is the X-Men's second mutant power.
Episodes that X-plain major retcons in the X-Men universe:
Retcons in generalEdit
The "death" of Jean GreyEdit
The Summers FamilyEdit
WRONG WAY GO BACK
Episodes that mention minor retcons:
Storylines featuring retcons recommended by listeners.