1 [U] extra help or support that you can get if necessary 额外的援助或支持；后备支援: The police officer requested urgent back-up from the rest of the team. 警察要求得到队里其他人员的紧急支持。

2 [C] (computing计算机技术) a copy of a computer disk that you can use if the original one is lost or damaged （计算机磁盘的）后备副本，备份: Always make a back-up of your files. 每次都要为计算机文件复制副本。