buttocks vs posterior

buttocks and posterior both are nouns.

Word	Noun	Adjective	Verb	Adverb
buttocks	Yes	No	No	No
posterior	Yes	Yes	No	No

As nouns, buttocks and posterior are synonyms defined as:

buttocks and posterior: the fleshy part of the human body that you sit on

buttocks (noun)	posterior (noun)
the fleshy part of the human body that you sit on	the fleshy part of the human body that you sit on
	a tooth situated at the back of the mouth

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