—n. 1. a staunch supporter or adherent; one who can be relied upon.
2. something upon which one can rely and therefore choose or use regularly.
3. 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.
4. 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.
5. on standby, in a state of readiness to act, respond, or be used immediately when needed.
—adj. 1. kept readily available for use in an emergency, shortage, or the like: a standby player. 2. of or pertaining to last-minute accommodations, the transport that offers them, or a traveler who is waiting for them: a standby flight. 3. of or pertaining to a waiting period. Also,stand' by".