Megiddo was a town situated on the Western edge of the Valley of Jezreel, the valley in which a decisive battle was fought when the Northern Kingdom of Israel (also called the house of Israel) was judged by God by being handed over to be conquered by Sargon II of Assyria (722 B.C), at which time the House of Israel became exiled and ceased being considered part of God's elect.
The best way to understand the meaning of the Greek word "Armageddon" is to consider the Battle of Waterloo that took place near the city of Waterloo in present-day Belgium (then part of The Kingdom of the Netherlands) in 1815. It was a decisive battle in which Napoleon was defeated.
It was not in Waterloo but near Waterloo where the battle took place.
"Armageddon" in Revelation 16:16 is the same as saying "Waterloo of the nations" and "Valley of Jehoshaphat", i.e the valley of "God will judge", (will judge the nations: Joel 3:12), because the word "Armageddon" like the word "Waterloo" came to be symbolic for decisive battles which, in the case of Armageddon, will take place in, and center around Jerusalem (Zechariah: Chapters 12 through 14; and Joel: Chapter 3).
Image below: Tel Megiddo: The ruins of Megiddo. The symbol of the decisive battle (a.k.a "Armageddon") which Zechariah chapters 12 through 14 tells us centers around Jerusalem:
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