Short for "retroactive continuity". When an author, usually of a long running series, decides to reframe some past event for the sake of the current story.
Like deciding in Spider-Man 3 that Sandman actually killed Peter Parker's Uncle back in Spider-Man 1.
The ret con backstory for Spike in Buffy was awesome. But trying to ret con Flint Marko into Spider-Man's origin is retarded.
to take previously known facts of a plot, then change the interpretation of it to fit new facts that are more curently convient to the writer, for no real reason.
Common occurrence on Angel and Buffy the Vampire Slayer