posterior vs backside

posterior and backside both are nouns.

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

As nouns, posterior and backside are synonyms defined as:

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

posterior (noun)	backside (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	the side of an object that is opposite its front

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