The Armageddon Clock is the name of the down counter of nuclear bomb drops remaining until the end of the world. That counter appears in the player(s) screen after the first nuclear bomb is detonated.
Armageddon is the term used to refer to the end of the known world, caused by nuclear warfare. In the game, it occurs when a certain number of Nuclear Missiles (determined by the number of players in the game) is launched. When this happens, the game ends, and all players lose the game with a score of 0. However, in some game modes (e.g.: set the resources to Infinite in a standard battle), Armageddon does not appear, so you are free to launch as many nukes as you like.
For each nation that researches the Missile Shield, the Armageddon Clock will be turned back by 2.
If the usage of nuclear missiles is allowed, CPU players will continuously use these destructive weapons until the world is about to reach Armageddon.