abdomen vs underbelly

abdomen and underbelly both are nouns.

Word	Noun	Adjective	Verb	Adverb
abdomen	Yes	No	No	No
underbelly	Yes	No	No	No

As nouns, underbelly is a hyponym of abdomen; that is, underbelly is a word with a more specific, narrower meaning than abdomen:

abdomen: the region of the body of a vertebrate between the thorax and the pelvis
underbelly: the soft belly or underside of an animal's body

Other hyponyms of abdomen include underbody.

abdomen (noun)	underbelly (noun)
the region of the body of a vertebrate between the thorax and the pelvis	the quality of being weak or unprotected
the cavity containing the major viscera; in mammals it is separated from the thorax by the diaphragm	the soft belly or underside of an animal's body
	lower side