Source : WordNet®

backup
     n 1: an accumulation caused by clogging or a stoppage; "a traffic
          backup on the main street"; "he discovered a backup in
          the toilet"
     2: someone who takes the place of another (as when things get
        dangerous or difficult); "the star had a stand-in for
        dangerous scenes"; "we need extra employees for summer
        fill-ins" [syn: {stand-in}, {substitute}, {relief}, {reliever},
         {backup man}, {fill-in}]
     3: a subordinate musical part; provides background for more
        important parts [syn: {accompaniment}, {musical
        accompaniment}, {support}]
     4: (computer science) a copy of a file or directory on a
        separate storage device; "he made a backup in case the
        original was accidentally damaged or erased" [syn: {computer
        backup}]
     5: the act of providing approval and support; "his vigorous
        backing of the conservatives got him in trouble with
        progressives" [syn: {backing}, {championship}, {patronage}]

Source : Free On-Line Dictionary of Computing

backup
     
         (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 {differential backup}, {incremental backup}, {full
        backup}.  Compare {archive}.
     
        (2003-06-22)