rear end vs body part

rear end and body part both are nouns.

Word	Noun	Adjective	Verb	Adverb
rear end	Yes	No	No	No
body part	Yes	No	No	No

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

rear end (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