a person or thing that can always be used if needed, for example if somebody/something else is not available or if there is an emergency 後備人員；備用物品
Idioms
on standby
ready to do something immediately if needed or asked 随時可以投入行動；處於待命狀態；招之即來 The emergency services were put on standby after a bomb warning. 接到炸彈警報後，各應急服務機構進入待命狀態。 As the level of the water rose, villages were put on evacuation standby. 随着水位上升，村莊處於疏散待命狀態。 The troops are on 24-hour standby (= ready to move at any time of the day or night). 部隊待命24小時（接到命令後24小時内可行動）。
ready to travel or go somewhere if a ticket or something that is needed suddenly becomes available 候補座位（如臨時有票或具備其他必要條件便可立即出發的）