island vs Niihau

island and Niihau both are nouns.

Word	Noun	Adjective	Verb	Adverb
island	Yes	No	No	No
Niihau	Yes	No	No	No

island (noun)	Niihau (noun)
a zone or area resembling an island	the most northwestern Hawaiian island (beyond Kauai)
a land mass (smaller than a continent) that is surrounded by water