"RETCON" is a shortening of the phrase "retroactive continuity".
It refers to a story or statement which in any way alters previously-established history
in an ongoing series.
A "retcon" can be as innocuous as Roy Thomas' All-Star Squadron (which coined the
phrase) or Kurt Busiek's Untold Tales of Spider-Man. These series simply tell
stories set in a particular past time, which don't contradict any of the previous stories.
For example, there was never an explanation in the 1960's of why Peter Parker stopped
wearing glasses; an Untold Tales of Spider-Man story explained how it happened.
At the other extreme, a "retcon" can be a complete dismissal of all past history.
Examples of this are John Byrne's Man of Steel mini-series in 1986, or the
Legion of Super-Heroes stories published after Zero Hour in August 1994.
Man of Steel retold the origin of Superman, implicitly declaring that all
previously-published stories about the character never happened, and that
previously-established facts were no longer necessarily true. For example, in the
1930's it was established that Ma and Pa Kent died when Clark was a young adult;
Man of Steel stated that they were still alive.
