The term "retcon" is short for "retroactive continuity". Applied to roleplaying, it refers to the rescinding of actions that occurred in roleplay to maintain the integrity of the game plot. This is considered an extreme measure, and is usually avoided as much as possible, but it can occasionally be necessary.
Basically, whenever there is a call for a retcon, all participants in a scene are asked to strike whatever has happened from their characters' memories, and the game then proceeds as if the retconned scene never happened. This may be used, for example, if the results of a roleplayed scene would result in consequences that are unacceptable to either the players or the admins, such as the death of a key character. Retcons can only be declared by the game admins, or by a unanimous agreement between all characters affected by a scene, assuming the scene took place in a private location.
On ShyeMUSH, retcons are exceedingly rare. More often, we prefer it if characters are forced to deal with the consequences of their IC actions. Therefore, it is highly advisable that you consider the possible IC consequences of your character's IC actions at all times.
Return to Game Guide