1832, from comb. form of Greek megas (genitive megalou) "great" (see mickle) + polis "city" (see polis). The word was used in classical times as an epithet of great cities (Athens, Syracuse, Alexandria), and it also was the name of a former city in Arcadia.
A vast stretch of developed industrial urban area, such as the East Coast of the United States from Boston to Washington, D.C., or the Ruhr Valley in Germany. Megalopolis is from Greek words meaning “great city.”