The word “Armageddon” in its popular usage has become the synonym for “doomsday” or “end of the world”. The word is found only one time in the Bible; “And they gathered them together to the place which in Hebrew is called Har-Magedon” (Revelation 16:16). It means the Mountain of Megiddo or the “Mount of Slaughter” (Har means mount and Megiddo means slaughter). Megiddo is a city in the northern part of the land of Israel. The city was built on a hill. The city of Megiddo overlooks the large valley of Jehoshaphat (also known as valley of Jezreel, the valley of Esdraelon, and the plains of Megiddo).
Though Armageddon is a place, in its biblical usage it mainly refers to the climatic event of the Great Tribulation, when all the armies of the earth gather to come to fight against Israel under the leadership of the Antichrist. It culminates in the glorious return of Jesus Christ, who will destroy the invading armies and deliver the faithful Jewish remnant (Revelation 16:12-16; 19:11-16; Joel 3:1-2; Zechariah 12:1-14; 14:1-4). The armies of the earth and the Antichrist will meet their doom in the battle of Armageddon in the plains of Megiddo. It will not be an individual battle, but a series of battles (the word translated war in Revelation 16:14 is the Greek word polemos which signifies a war or campaign).
Armageddon is not the same as the battle of Gog and Magog in Ezekiel 38-39 and it is not the final war on earth. The final war is the revolt of Satan mentioned in Revelation 20:7-10. Some people refer to Armageddon as World War III (?).