posterior vs retral

posterior is a noun but retral is not a noun.

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

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

posterior (adjective)	retral (adjective)
located at or near or behind a part or near the end of a structure	moving or directed or tending in a backward direction or contrary to a previous direction
coming at a subsequent time or stage	at or near or toward the posterior