standby

名词 standby:

1. something that can be relied on when needed
2. an actor able to replace a regular performer when required
   同义词：understudy

形容词 standby:

1. ready for emergency use

1. A special team of police were kept on standby during the time of violence.
   在发生暴乱期间，经常有一支警察特别小分队处于整装待命状态。