Niihau Island vs Niihau

Niihau Island and Niihau both are nouns.

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

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

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

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