island vs islet

island and islet both are nouns.

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

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

Other hyponyms of island include Caribbean Island, Aegean island, barrier island, isle, South Sea Islands.

island (noun)	islet (noun)
a zone or area resembling an island	a small island
a land mass (smaller than a continent) that is surrounded by water