armpit vs axillary fossa

armpit and axillary fossa both are nouns.

Word	Noun	Adjective	Verb	Adverb
armpit	Yes	No	No	No
axillary fossa	Yes	No	No	No

As nouns, armpit and axillary fossa are synonyms defined as:

armpit and axillary fossa: the hollow under the arm where it is joined to the shoulder

Other synonyms of armpit include axilla, axillary cavity.

armpit (noun)	axillary fossa (noun)
the hollow under the arm where it is joined to the shoulder	the hollow under the arm where it is joined to the shoulder