1. In information technology, a backup, or the process of backing up, refers to the copying and archiving of computer data so it may be used to restore the original after a data loss event. Backups have two distinct purposes. The primary purpose is to recover data after its loss, be it by data deletion or corruption. The secondary purpose of backups is to recover data from an earlier time, according to a user-defined data retention policy, typically configured within a backup application for how long copies of data are required.
2. In general, a backup is someone or something that provides support or help, in case the main plans, equipment etc. go wrong. In computing systems, a backup is a copy of information held on a computer that is stores separately.
https://en.wikipedia.org/wiki/Backup (1.); http://dictionary.cambridge.org/dictionary/english/backup (2.)