island vs terra firma

island and terra firma both are nouns.

Word	Noun	Adjective	Verb	Adverb
island	Yes	No	No	No
terra firma	Yes	No	No	No

As nouns, terra firma is a hypernym of island; that is, terra firma is a word with a broader meaning than island:

Other hypernyms of island include dry land, earth, ground, land, solid ground.

island (noun)	terra firma (noun)
a zone or area resembling an island	the solid part of the earth's surface
a land mass (smaller than a continent) that is surrounded by water