Backup consists of extra equipment, resources or people that you can get help or support from if necessary.
双语例句
例：
There is no emergency back-up immediately available.
没有能立即获得的后备。
2.?[C, U 有变体名词] 备用物（如设备、计划）
If you have something such as a second piece of equipment or set of plans as backup, you have arranged for them to be available for use in case the first one does not work.