pay vs pay back

pay is a noun but pay back is not a noun.

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

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

pay (verb)	pay back (verb)
dedicate	take vengeance on or get even
bear (a cost or penalty), in recompense for some action	act or give recompense in recognition of someone's behavior or actions
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