archipelago vs terra firma

archipelago and terra firma both are nouns.

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

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

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

archipelago (noun)	terra firma (noun)
a group of many islands in a large body of water	the solid part of the earth's surface