Saint Petersburg vs city

Saint Petersburg and city both are nouns.

Word	Noun	Adjective	Verb	Adverb
Saint Petersburg	Yes	No	No	No
city	Yes	No	No	No

Saint Petersburg (noun)	city (noun)
a city in the European part of Russia; 2nd largest Russian city; located at the head of the Gulf of Finland; former capital of Russia	people living in a large densely populated municipality
a city in western Florida on Tampa Bay; a popular winter resort	a large and densely populated urban area; may include several independent administrative districts
	an incorporated administrative district established by state charter