What’s a Retcon?

I was recently reading something related to the new X-men film, and encountered the term retcon. It refers to comic book continuity, and apparently works both as a noun and a verb:

Retroactive continuity–commonly contracted to the blend retcon–is the adding of new information to “historical” material, or deliberately changing previously established facts in a work of serial fiction. The change itself is referred to as a “retcon”, and the act of writing and publishing a retcon is called “retconning”.

The afore-linked (heh) Wikipedia article has plenty of examples. Everything from She-Hulk to Roseanne has been retconned at one time or another.