A.T. Robertson Commentary Revelation 17:18

"And the woman whom thou sawest is the great city, which reigneth over the kings of the earth." — Revelation 17:18 (ASV)

The woman (η γυνη). She is now explained after the beast has been interpreted. Verse 9 made it plain enough, but this verse demonstrates that the woman is the city of Rome "which reigneth (η εχουσα βασιλειαν, the one having a kingdom) over the kings of the earth (επ των βασιλεων της γης)." Rome followed Babylon, and other cities may follow in their train.