back·up / ˈbakˌəp/ • n. 1. help or support: no police backup could be expected. ∎  a person or thing that can be called on if necessary; a reserve: I've got a security force as backup [as adj.] a backup generator. 2. Comput. the procedure for making extra copies of data in case the original is lost or damaged: automatic online backup | [as adj.] a backup system. ∎  a copy of this type.3. an overflow caused by a stoppage, as in water or automobile traffic: there are long backups on all routes.

backup A resource that is, or can be used as, a substitute when a primary resource fails or when a file has been corrupted. The word is also used as a verb, to back up, i.e. to make a copy in anticipation of future failure or corruption. Thus a dump forms a backup to be used in cases where a user's file has become unusable; the taking of the dump can be regarded as backing up the version on disk.