a staunch supporter or adherent; one who can be relied upon.

something upon which one can rely and therefore choose or use regularly.

something or someone held ready to serve as a substitute, esp. a radio or television program used as a filler in case of cancellation of a regularly scheduled program.

a traveler who is waiting for last-minute accommodations to become available on a plane, train, or other transport as a result of a cancellation.

kept readily available for use in an emergency, shortage, or the like

a standby player.

of or pertaining to last-minute accommodations, the transport that offers them, or a traveler who is waiting for them

a standby flight.

of or pertaining to a waiting period. Also, stand by. ?

on standby, in a state of readiness to act, respond

or be used immediately when needed.

noun

assistant , backup , deputy , double , fill-in , pinch-hitter , relief , replacement , reserve , stalwart , stand-in , sub * , successor , temp , temporary , understudy

adjective

backup , emergency , reserve , secondary , supplemental , supplementary