posterior vs later

posterior is a noun but later is not a noun.

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

As adjectives, posterior and later are synonyms defined as:

Other synonyms of posterior include ulterior.

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

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

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