great power vs major power

great power and major power both are nouns.

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

As nouns, great power and major power are synonyms defined as:

great power and major power: a state powerful enough to influence events throughout the world

Other synonyms of great power include power, superpower, world power.

great power (noun)	major power (noun)
a state powerful enough to influence events throughout the world	a state powerful enough to influence events throughout the world