tail bone vs os

tail bone and os both are nouns.

Word	Noun	Adjective	Verb	Adverb
tail bone	Yes	No	No	No
os	Yes	No	No	No

As nouns, os is a hypernym of tail bone; that is, os is a word with a broader meaning than tail bone:

Other hypernyms of tail bone include bone.

tail bone (noun)	os (noun)
the end of the vertebral column in humans and tailless apes	rigid connective tissue that makes up the skeleton of vertebrates
	a mouth or mouthlike opening