() a copy of a file, program, etc. that can be used if the original is lost or damaged （文件、程序等的）备份◆Always make a backup of your work. 你的工作文件要经常进行备份。◆a backup copy 备份文件
extra help, ideas, equipment, etc. that you can use if the first fails 增援；替代设备◆We have a backup plan if the merger falls through. 如果合并落空，我们还有後备计划。