reply vs feed back

reply is a noun but feed back is not a noun.

Word	Noun	Adjective	Verb	Adverb
reply	Yes	No	Yes	No
feed back	No	No	Yes	No

reply (noun)	feed back (noun)
the speech act of continuing a conversational exchange
a statement (either spoken or written) that is made to reply to a question or request or criticism or accusation

reply (verb)	feed back (verb)
react verbally	respond to a query or outcome
	submit (information) again to a program or automatic system