urban center vs Istanbul

urban center and Istanbul both are nouns.

Word	Noun	Adjective	Verb	Adverb
urban center	Yes	No	No	No
Istanbul	Yes	No	No	No

urban center (noun)	Istanbul (noun)
a large and densely populated urban area; may include several independent administrative districts	the largest city and former capital of Turkey; rebuilt on the site of ancient Byzantium by Constantine I in the fourth century; renamed Constantinople by Constantine who made it the capital of the Byzantine Empire; now the seat of the Eastern Orthodox Church