isthmus vs terra firma

isthmus and terra firma both are nouns.

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

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

isthmus: a relatively narrow strip of land (with water on both sides) connecting two larger land areas
terra firma: the solid part of the earth's surface

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

isthmus (noun)	terra firma (noun)
a cord-like tissue connecting two larger parts of an anatomical structure	the solid part of the earth's surface
a relatively narrow strip of land (with water on both sides) connecting two larger land areas