elbow bone vs ulna

elbow bone and ulna both are nouns.

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

As nouns, elbow bone and ulna are synonyms defined as:

elbow bone and ulna: the inner and longer of the two bones of the human forearm

elbow bone (noun)	ulna (noun)
the inner and longer of the two bones of the human forearm	the inner and longer of the two bones of the human forearm