posterior vs tail end

posterior and tail end both are nouns.

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

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

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

posterior (noun)	tail 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	any projection that resembles the tail of an animal
	the time of the last part of something

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