A copy of files and programs made to facilitate recovery, if necessary. (NIST SOURCE: SP 800-34; CNSSI-4009)
Copying data to ensure its availability in the case of computer failure or loss. (www.getsafeonline.org/jargon-buster)
Backup falls under the data & recovery cells in the Sounil Yu Matrix.
(Also see Backup Site, Backup Facility [As per "Hot", "Cold", & Warm Site])