standby

One that can always be relied on, as in an emergency.

One kept in readiness to serve as a substitute.

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.

Kept in reserve for use when needed.

Of, relating to, or waiting for unreserved travel space that is made available by an airline only shortly before departure.

On a standby basis.

(on standby) Ready and available.

(on standby) As a standby passenger.

State of readiness without being immediately involved.

Waiting at the airport in the hope of getting a seat on a flight that is already booked out.