South Sea Islands vs island

South Sea Islands and island both are nouns.

Word	Noun	Adjective	Verb	Adverb
South Sea Islands	Yes	No	No	No
island	Yes	No	No	No

As nouns, island is a hypernym of South Sea Islands; that is, island is a word with a broader meaning than South Sea Islands:

South Sea Islands: any islands in the southern or southwestern of central parts of the Pacific Ocean
island: a land mass (smaller than a continent) that is surrounded by water

South Sea Islands (noun)	island (noun)
any islands in the southern or southwestern of central parts of the Pacific Ocean	a zone or area resembling an island
	a land mass (smaller than a continent) that is surrounded by water