stand-by
noun
Definitions
A state of readiness without immediate involvement; remaining in preparation for (a sudden or unforeseen event or situation).
Example: The troops were on standby in case of an attack.
Sleep mode
(travel) Waiting at the airport in the hope of getting a seat on a flight that is already booked out.
Something that is standard, well-tested, or frequently used.
Example: That recipe is an old standby, and she keeps the ingredients around in case of unexpected guests.