back tooth vs posterior

back tooth and posterior both are nouns.

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

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

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

back tooth (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