later vs posterior

later is not a noun while posterior is a noun.

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

As adjectives, later and posterior are synonyms defined as:

Other synonyms of later include ulterior.

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

later (adjective)	posterior (adjective)
at or toward an end or late period or stage of development	located at or near or behind a part or near the end of a structure
coming at a subsequent time or stage	coming at a subsequent time or stage

later (adverb)	posterior (adverb)
happening at a time subsequent to a reference time
at some eventual time in the future
comparative of the adverb `late'