long bone vs diaphysis

long bone and diaphysis both are nouns.

Word	Noun	Adjective	Verb	Adverb
long bone	Yes	No	No	No
diaphysis	Yes	No	No	No

As nouns, diaphysis is a hyponym of long bone; that is, diaphysis is a word with a more specific, narrower meaning than long bone:

long bone: in limbs of vertebrate animals: a long cylindrical bone that contains marrow
diaphysis: the main (mid) section of a long bone

Other hyponyms of long bone include epiphysis, shaft, arm bone, leg bone.

long bone (noun)	diaphysis (noun)
in limbs of vertebrate animals: a long cylindrical bone that contains marrow	the main (mid) section of a long bone