armpit vs cavity

armpit and cavity both are nouns.

Word	Noun	Adjective	Verb	Adverb
armpit	Yes	No	No	No
cavity	Yes	No	No	No

As nouns, cavity is a hypernym of armpit; that is, cavity is a word with a broader meaning than armpit:

Other hypernyms of armpit include bodily cavity, cavum.

armpit (noun)	cavity (noun)
the hollow under the arm where it is joined to the shoulder	(anatomy) a natural hollow or sinus within the body
	a sizeable hole (usually in the ground)
	soft decayed area in a tooth; progressive decay can lead to the death of a tooth
	space that is surrounded by something