arm bone vs ulna

arm bone and ulna both are nouns.

Word	Noun	Adjective	Verb	Adverb
arm bone	Yes	No	No	No
ulna	Yes	No	No	No

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

Other hyponyms of arm bone include humerus, radius, elbow bone.

arm bone (noun)	ulna (noun)
a bone in the arm	the inner and longer of the two bones of the human forearm