Niihau vs Niihau Island

Niihau and Niihau Island both are nouns.

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

As nouns, Niihau and Niihau Island are synonyms defined as:

Niihau and Niihau Island: the most northwestern Hawaiian island (beyond Kauai)

Niihau (noun)	Niihau Island (noun)
the most northwestern Hawaiian island (beyond Kauai)	the most northwestern Hawaiian island (beyond Kauai)