city vs St. Petersburg

city and St. Petersburg both are nouns.

Word	Noun	Adjective	Verb	Adverb
city	Yes	No	No	No
St. Petersburg	Yes	No	No	No

city (noun)	St. Petersburg (noun)
people living in a large densely populated municipality	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
a large and densely populated urban area; may include several independent administrative districts	a city in western Florida on Tampa Bay; a popular winter resort
an incorporated administrative district established by state charter