buttock vs body part

buttock and body part both are nouns.

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

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

buttock: either of the two large fleshy masses of muscular tissue that form the human rump
body part: any part of an organism such as an organ or extremity

buttock (noun)	body part (noun)
either of the two large fleshy masses of muscular tissue that form the human rump	any part of an organism such as an organ or extremity