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