South Island vs island

South Island and island both are nouns.

Word	Noun	Adjective	Verb	Adverb
South Island	Yes	No	No	No
island	Yes	No	No	No

South Island (noun)	island (noun)
the larger but less populous of two main islands of New Zealand; separated from North Island by Cook Strait	a zone or area resembling an island
	a land mass (smaller than a continent) that is surrounded by water