pit of the stomach vs fossa

pit of the stomach and fossa both are nouns.

Word	Noun	Adjective	Verb	Adverb
pit of the stomach	Yes	No	No	No
fossa	Yes	No	No	No

As nouns, fossa is a hypernym of pit of the stomach; that is, fossa is a word with a broader meaning than pit of the stomach:

pit of the stomach: a slight depression in the midline just below the sternum (where a blow can affect the solar plexus)
fossa: a concavity in a surface (especially an anatomical depression)

Other hypernyms of pit of the stomach include pit.

pit of the stomach (noun)	fossa (noun)
a slight depression in the midline just below the sternum (where a blow can affect the solar plexus)	largest carnivore of Madagascar; intermediate in some respects between cats and civets
	a concavity in a surface (especially an anatomical depression)