n. One that can always be relied on, as in an emergency.n. A favorite or frequent choice.n. One kept in readiness to serve as a substitute.adj. Kept in reserve for use when needed: a standby generator. adj. Of, relating to, or waiting for unreserved travel space that is made available by an airline only shortly before departure: standby passengers. ad. On a standby basis: flew standby to New York. idiom. on standby Ready and waiting.