posterior vs back tooth

posterior and back tooth both are nouns.

Word	Noun	Adjective	Verb	Adverb
posterior	Yes	Yes	No	No
back tooth	Yes	No	No	No

As nouns, posterior and back tooth are synonyms defined as:

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

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