stand by vs standby

stand by is not a noun while standby is a noun.

Word	Noun	Adjective	Verb	Adverb
stand by	No	No	Yes	No
standby	Yes	Yes	No	No

stand by (noun)	standby (noun)
	something that can be relied on when needed
	an actor able to replace a regular performer when required

stand by (adjective)	standby (adjective)
	ready for emergency use

stand by (verb)	standby (verb)
not act or do anything
be loyal to
be available or ready for a certain function or service