repay vs pay back

repay and pay back both are verbs.

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

As verbs, repay and pay back are synonyms defined as:

repay and pay back: act or give recompense in recognition of someone's behavior or actions

Other synonyms of repay include reward.

repay (verb)	pay back (verb)
answer back	take vengeance on or get even
pay back	act or give recompense in recognition of someone's behavior or actions
make repayment for or return something
act or give recompense in recognition of someone's behavior or actions