I. stand‧by¹ , ˈstand-by /ˈstændbaɪ/ noun (plural standbys)

1. on standby
a) ready to help immediately if you are needed:
A special team of police were kept on standby.
b) if you are on standby to do something, for example to travel by plane, you are on a list of people who may be allowed to do it if places become available:
We can put you on standby. ⇒ stand by(4)

2. [countable] something that is kept ready so that it can be used when needed:
Powdered milk is a good standby in an emergency.

[TahlilGaran] Dictionary of Contemporary English ▲

II. standby² adjective [only before noun]
a standby ticket is one that you can get only if places become available, for example if other people cannot use their tickets

[TahlilGaran] Dictionary of Contemporary English ▲