belly vs underbody

belly and underbody both are nouns.

Word	Noun	Adjective	Verb	Adverb
belly	Yes	No	Yes	No
underbody	Yes	No	No	No

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

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

Other hyponyms of belly include underbelly.

belly (noun)	underbody (noun)
the underpart of the body of certain vertebrates such as snakes or fish	the soft belly or underside of an animal's body
the region of the body of a vertebrate between the thorax and the pelvis
a protruding abdomen
the hollow inside of something
a part that bulges deeply

belly (verb)	underbody (verb)
swell out or bulge out