standby

Short definition from Wiktionary

Noun

1. 1
   A state of readiness without immediate involvement; remaining in preparation for (a sudden or unforeseen event or situation).
2. 2
   A replacement kept on standby.
3. 3
   Sleep mode.
4. 4
   Waiting at the airport in the hope of getting a seat on a flight that is already booked out.
5. 5
   Something that is standard, well-tested, or frequently used.

Verb

1. 1
   To wait briefly, as for additional communication by radio or telephone; alternative form of stand by.