city centre vs metropolis

city centre and metropolis both are nouns.

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

city centre (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