pay back vs pay

pay back is not a noun while pay is a noun.

Word	Noun	Adjective	Verb	Adverb
pay back	No	No	Yes	No
pay	Yes	No	Yes	No

pay back (noun)	pay (noun)
	something that remunerates

pay back (verb)	pay (verb)
take vengeance on or get even	dedicate
act or give recompense in recognition of someone's behavior or actions	bear (a cost or penalty), in recompense for some action
	convey, as of a compliment, regards, attention, etc.; bestow
	give money, usually in exchange for goods or services
	bring in
	do or give something to somebody in return
	cancel or discharge a debt
	make a compensation for
	discharge or settle
	render
	be worth it