backside vs posterior

backside and posterior both are nouns.

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

As nouns, backside and posterior are synonyms defined as:

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

backside (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
the side of an object that is opposite its front	a tooth situated at the back of the mouth

backside (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