Polynesian vs Tahitian

Polynesian and Tahitian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Polynesian	Yes	Yes	No	No
Tahitian	Yes	Yes	No	No

As nouns, Tahitian is a hyponym of Polynesian; that is, Tahitian is a word with a more specific, narrower meaning than Polynesian:

Other hyponyms of Polynesian include Fijian, Samoan, Tongan.

Polynesian (noun)	Tahitian (noun)
the branch of the Austronesian languages spoken from Madagascar to the central Pacific	the Oceanic language spoken on Tahiti
a native or inhabitant of Polynesia	a native or inhabitant of Tahiti

Polynesian (adjective)	Tahitian (adjective)
of or relating to Polynesia or its people or culture	of or relating to or characteristic of the island of Tahiti or its residents or their language and culture