Armageddon (lit. har "mount" and "megiddo") is the location of the final great battle between good and evil called the Great Day of God Almighty. The word, "armageddon," only occurs in Rev. 16:16. It is also spelled Harmagedon. Many armies will gather there from all over--possibly, China, Iran, Iraq, Russia, and more.
Megiddo is located between Mt. Carmel and Mt. Gilboa and about 15 x 15 x 20 miles. It is a large flat area.