n. pl. stand·bys 1. One that can always be relied on, as in an emergency.
2. One kept in readiness to serve as a substitute.
3. A mode of operation for a computer, television, or other electronic device in which the power is on and the device is ready for immediate use.
adj.1. Kept in reserve for use when needed: a standby generator.
2. Of, relating to, or waiting for unreserved travel space that is made available by an airline only shortly before departure: standby passengers.