Armageddon

The wicked cry alack, alas When Jesus comes to kick their ass. With a full hogshead of pure whoop-ass, The whole wide world will be ta'en to task. In Christian mythology, Armageddon is the term for the final battle between the forces of God and Satan. Different Christian denominations teach variously on what exactly would happen then.

Biblical use
The name specifically refers to the place at which the final eschatological battle would take place, the Hill of Megiddo (הר מגידו, har megiddo). Several Old Testament battles allegedly took place around Megiddo. Some Christians believe that a literal battle will take place at or near Megiddo while others see the relevant text as metaphorical.

Armageddon is more commonly (but slightly incorrectly) used as a Biblical term for the actual battle/event between the forces of God and Satan. Rare planetary alignments may indicate the start of events leading up to Armageddon, notably the rise of Antichrist.

Common use
Figuratively, any battle that would bring about the end of the world can be considered an "Armageddon". With the advent of nuclear weapons and the threat of nuclear war, this has become a real possibility, one that keeps many defense analysts awake during the night and well-paid during the day. It is sometimes used for natural causes of the end of the world, such as in...

Movie
Armageddon is also the name of a 1998 sci-fi action film by Michael Bay, which details a fictional account of a potential asteroid impact. It is so notorious for its utter defilement of science that NASA uses it in their management training program, challenging staff to see how many inaccuracies they can spot. And by "inaccuracies", we're not just talking about things that are unlikely to happen in real life — we mean stuff that's downright impossible. In the words of Phil Plait, "Armageddon got some astronomy right. For example, there is an asteroid in the movie, and asteroids do indeed exist. And then there was... um... well, you know... um. Okay, so that was about all they got right."