Standby

readiness for duty or immediate deployment: buses were placed on standby for the journey to London.

• [ count noun ] a person or thing ready to be deployed immediately, especially if needed as backup in an emergency: a generator was kept as a standby in case of power failure | [ as modifier ] : a standby rescue vessel.
• an operational mode of an electrical appliance in which the power is switched on but the appliance is not actually functioning: switch off the TV at night instead of leaving it on standby.