ulterior vs posterior

ulterior is not a noun while posterior is a noun.

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

As adjectives, ulterior and posterior are synonyms defined as:

Other synonyms of ulterior include later.

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

ulterior (adjective)	posterior (adjective)
lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed)	located at or near or behind a part or near the end of a structure
beyond or outside an area of immediate interest; remote	coming at a subsequent time or stage
coming at a subsequent time or stage