back up vs backup

back up is not a noun while backup is a noun.

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

back up (noun)	backup (noun)
	the act of providing approval and support
	(computer science) a copy of a file or directory on a separate storage device
	a musical part (vocal or instrumental) that supports or provides background for other musical parts
	someone who takes the place of another (as when things get dangerous or difficult)
	an accumulation caused by clogging or a stoppage

back up (verb)	backup (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