long bone vs os longum

long bone and os longum both are nouns.

Word	Noun	Adjective	Verb	Adverb
long bone	Yes	No	No	No
os longum	Yes	No	No	No

As nouns, long bone and os longum are synonyms defined as:

long bone and os longum: in limbs of vertebrate animals: a long cylindrical bone that contains marrow

long bone (noun)	os longum (noun)
in limbs of vertebrate animals: a long cylindrical bone that contains marrow	in limbs of vertebrate animals: a long cylindrical bone that contains marrow