computer vs back up

computer is a noun but back up is not a noun.

Word	Noun	Adjective	Verb	Adverb
computer	Yes	No	No	No
back up	No	No	Yes	No

computer (noun)	back up (noun)
a machine for performing calculations automatically
an expert at calculation (or at operating calculating machines)

computer (verb)	back up (verb)
	establish as valid or genuine
	become or cause to become obstructed
	make a copy of (a computer file) especially for storage in another place as a security copy
	move backwards from a certain position
	give moral or psychological support, aid, or courage to