city centre vs city

city centre and city both are nouns.

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

city centre (noun)	city (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
	an incorporated administrative district established by state charter