central city vs metropolis

central city and metropolis both are nouns.

Word	Noun	Adjective	Verb	Adverb
central city	Yes	No	No	No
metropolis	Yes	No	No	No

central city (noun)	metropolis (noun)
the central part of a city	people living in a large densely populated municipality
	a large and densely populated urban area; may include several independent administrative districts