stand-by Definition

stand-by
noun stand-by's, stand-bys

1a. A state of readiness to act, eg in an emergency;
1b. A person or thing that takes on this kind of role.
2a. Said of air travel: a system of allocating spare seats to passengers who do not have reservations, after all the booked seats have been taken;
2b. A ticket that has been allocated in this way.

Idiom: on stand-by

Ready and prepared to do something if necessary.
Example: The emergency team were on stand-by