The word “Armageddon” is really “Har-Magedon” in the Greek. Revelation 16:16 explains that the word is based on the Hebrew. In Hebrew “Har” means mountain. So the most natural meaning of Armageddon is “Mountain of Megiddo.” The problem with that reading is that there is no mountain in the whole world named Megiddo. The Bible refers to the waters of Megiddo (Jdg. 5:19), a Valley of Megiddo (2 Chr. 35:22) and a city of Megiddo (1 Kgs. 9:15). But nowhere is there a reference to a mountain of Megiddo.
There are a couple of other possibilities. In Zechariah 12:11 the LXX translator translates the Hebrew for “Megiddo” with “slaughter.” The mourning of Jerusalem in the future is compared to the mourning “of” or “for” Haddad-rimmon. We don’t know who or what Haddad-rimmon was, it is a Syrian name and the event referred to occurred outside Scripture. Relevant to our purpose, the mourning, whatever it refers to, is not in the city of Megiddo, but in a place where slaughter occurred. If this is what the author of Revelation had in mind, “Armageddon” would be a reference to Zechariah 12:11, and would mean “mountain of slaughter.”
Another option suggests that “Armageddon” is a reference to the fall of Lucifer in Isaiah 14. Lucifer fell from the “Mount of Assembly” (echoing Isa 14:12). But the expression in the Hebrew of Isaiah 14:12 is quite different from that of Revelation.
The Anchor Bible Dictionary concludes that the best explanation of “Har-Magedon” is to associate it with the mountain that looms over the waters, valley and city of Megiddo; Mount Carmel. Mount Carmel is the place where Elijah called fire down from heaven to earth to demonstrate who the true God is (Rev. 13:13-14). If this was John’s intent, in the last days of earth’s history there will be a showdown between the true God (Rev. 4-5) and the counterfeit trio (Rev. 16:13-14), between the three angels (Rev. 14:6-12) and the three frogs. In that final contest, the fire will fall on the wrong altar (Rev. 13:13-14), but the true God will be vindicated in the end (Rev. 15:3-4).