pay back vs payback

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

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

pay back (noun)	payback (noun)
	the act of taking revenge (harming someone in retaliation for something harmful that they have done) especially in the next life
	financial return or reward (especially returns equal to the initial investment)

pay back (verb)	payback (verb)
take vengeance on or get even
act or give recompense in recognition of someone's behavior or actions