Dominion vs res publica

Dominion and res publica both are nouns.

Word	Noun	Adjective	Verb	Adverb
Dominion	Yes	No	No	No
res publica	Yes	No	No	No

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

Dominion: one of the self-governing nations in the British Commonwealth
res publica: a politically organized body of people under a single government

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

Dominion (noun)	res publica (noun)
one of the self-governing nations in the British Commonwealth	a politically organized body of people under a single government