island vs Bougainville Island

island and Bougainville Island both are nouns.

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

island (noun)	Bougainville Island (noun)
a zone or area resembling an island	the largest of the Solomon Islands; a province of Papua New Guinea
a land mass (smaller than a continent) that is surrounded by water