great power vs nation

great power and nation both are nouns.

Word	Noun	Adjective	Verb	Adverb
great power	Yes	No	No	No
nation	Yes	No	No	No

As nouns, nation is a hypernym of great power; that is, nation is a word with a broader meaning than great power:

great power: a state powerful enough to influence events throughout the world
nation: a politically organized body of people under a single government

Other hypernyms of great power include body politic, commonwealth, country, land, res publica, state.

great power (noun)	nation (noun)
a state powerful enough to influence events throughout the world	a politically organized body of people under a single government
	the people who live in a nation or country
	a federation of tribes (especially Native American tribes)