(Sometimes "back up" when used as a verb) A spare copy of a file, file system or other resource for use in the event of failure or loss of the original.

The term is most commonly used to refer to a copy of all the files on a computer's disks which is made periodically and kept on magnetic tape or other removable medium (also called a "dump").

This essential precaution is neglected by most new computer users until the first time they experience a disk crash or accidentally delete the only copy of the file they have been working on for the last six months.

Ideally the backup copies should be kept at a different site or in a fire safe since, though your hardware may be insured against fire, the data on it is almost certainly neither insured nor easily replaced.

See also incremental backup. Compare archive.