<p><strong>Noun</strong></p><ul><li>a staunch supporter or adherent; one who can be relied upon.</li><li>something upon which one can rely and therefore choose or use regularly.</li><li>something or someone held ready to serve as a substitute, especially a radio or television program used as a filler in case of cancellation of a regularly scheduled program.</li><li>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.</li></ul><p><strong>Adjective</strong></p><ul><li>kept readily available for use in an emergency, shortage, or the like: a standby player.</li><li>of or relating to last-minute accommodations, the transport that offers them, or a traveler who is waiting for them: a standby flight.</li><li>of or relating to a waiting period.</li></ul>