stand-by
n (pl -bys)
1 person or thing available as a substitute or in an emergency 後备人员; 备用的事物
Aspirin is a good stand-by for headaches. 阿司匹林是必备的头痛良药.
[attrib 作定语]
a stand-by ticket, ie a cheaper type of airline ticket available when not all the tickets for a flight have been sold 剩余机票（因未事先售出而价廉）.
2 (idm 习语) on `stand-by in a state of readiness 待命状态
The troops are on 24-hour stand-by, ie ready to move within 24 hours of receiving the order. 部队待命24小时（接到命令後24小时内可行动）.