posterior vs rear end

posterior and rear end both are nouns.

Word	Noun	Adjective	Verb	Adverb
posterior	Yes	Yes	No	No
rear end	Yes	No	No	No

As nouns, posterior and rear end are synonyms defined as:

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

posterior (noun)	rear end (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

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