posterior vs body part

posterior and body part both are nouns.

Word	Noun	Adjective	Verb	Adverb
posterior	Yes	Yes	No	No
body part	Yes	No	No	No

As nouns, body part is a hypernym of posterior; that is, body part is a word with a broader meaning than posterior:

posterior (noun)	body part (noun)
the fleshy part of the human body that you sit on	any part of an organism such as an organ or extremity
a tooth situated at the back of the mouth

posterior (adjective)	body part (adjective)
located at or near or behind a part or near the end of a structure
coming at a subsequent time or stage