One or more copies of a computer file kept on one or more types of external (or internal) media as a safeguard against accidental deletion or damage of the original file. One can never have too many backups. Also known as a backup copy or backup file.
Backup also refers to any duplicate apparatus to be used in case of accident to the primary unit(s). Sometimes spelled as two words, back up, or is hyphenated, back-up. See also Differential Backup or Incremental Backup.