In Christian apocalyptic literature (the Book of Revelation), Armageddon is the site of the final battle between the kings of the earth (incited by Satan) and the Christian God. The term is also used for the battle itself, or more broadly for an Apocalypse.
The word armageddon is derived from Mount (Har in Hebrew) Megiddo, the site of the Battle of Megiddo and other battles.
Armageddon also refers to any great loss of life in battle etc.
See also: Armageddon (movie)