standby vs stand by

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

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

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

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

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