San Diego vs city

San Diego and city both are nouns.

Word	Noun	Adjective	Verb	Adverb
San Diego	Yes	No	No	No
city	Yes	No	No	No

San Diego (noun)	city (noun)
a picturesque city of southern California on San Diego Bay near the Mexican border; site of an important naval base	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