city state vs res publica

city state and res publica both are nouns.

Word	Noun	Adjective	Verb	Adverb
city state	Yes	No	No	No
res publica	Yes	No	No	No

As nouns, res publica is a hypernym of city state; that is, res publica is a word with a broader meaning than city state:

city state: a state consisting of a sovereign city
res publica: a politically organized body of people under a single government

Other hypernyms of city state include body politic, commonwealth, country, land, nation, state.

city state (noun)	res publica (noun)
a state consisting of a sovereign city	a politically organized body of people under a single government