Dictionary.net

an accumulation caused by clogging or a stoppage; "a traffic backup on the main street"; "he discovered a backup in the toilet"

("back up" when used as a verb) A sparecopy of a file, file system, or other resource for use in theevent of failure or loss of the original.The term commonly refers to a copy of the files on acomputer's disks, made periodically and kept on magnetictape or other removable medium (also called a "dump").This essential precaution is neglected by most new computerusers until the first time they experience a disk crash oraccidentally delete the only copy of the file they have beenworking on for the last six months. Ideally the backup copiesshould be kept at a different site or in a fire safe since,though your hardware may be insured against fire, the data onit is almost certainly neither insured nor easily replaced.See also backup software, differential backup,incremental backup, full backup. Compare archive,source code management.