standby
n. C / Un. something or someone that is ready to be used if the main choice is not available. It also describes a mode where an electronic device is waiting to be turned on quickly.
n. a person or thing that can be relied upon in an emergency or when the primary option fails. In electronics, refers to a power mode that allows for near-instantaneous activation.
I always keep some canned soup as a standby for quick meals.
The airline told the passengers they were on standby for the next available flight to London.
While the primary generator is undergoing its annual maintenance, the hospital relies on a secondary standby system to ensure that life-support machines remain operational during any potential power surges.
Deverbal from stand by.
Often used in the phrase 'on standby' to indicate readiness for immediate use or action.