The Collaborative International Dictionary of English v.0.48:

backup \back"up\, n.
   1. anything kept in reserve to serve as a substitute in case
      of failure or unavailability of the normal or primary
      object; -- used for devices, plans, people, etc. Also used
      attributively; as, there was no backup for the electrical
      supply; a backup motor; a backup generator.
      [PJC]

   2. (Music) a musician or group of musicians accompanying a
      soloist, whether vocalists or instrumentalists.

   Syn: accompaniment. [PJC]

   3. a person or group of persons serving as reinforcement for
      another or others; as, the policeman called for backup
      when he was fired on.
      [PJC]

   4. an accumulation, overflow, or reverse flow (in traffic or
      a liquid flow system) caused by a stoppage or other
      malfunction.
      [PJC]

   5. (Computers) A copy of a program or data from a computer
      onto a data-storage medium, usually one that may be
      removed to safe storage at a distance from the computer;
      it is used to preserve data for use in the contingency
      that the original data on the computer may be lost or
      become unusable. A backup that is removed from the
      building housing the computer system (to provide
      protection against loss of data in a disastrous event such
      as a fire) is called off-site backup. Also used
      attributively; as, backup copy.
      [PJC]

   6. (Computers) The act or process of creating a backup[5];
      as, they performed a full system backup every weekend.
      [PJC] Backward

WordNet (r) 3.0 (2006):

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, backup man, fill-in]
    3: a musical part (vocal or instrumental) that supports or
       provides background for other musical parts [syn:
       accompaniment, musical accompaniment, backup,
       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: backup,
       computer backup]
    5: the act of providing approval and support; "his vigorous
       backing of the conservatives got him in trouble with
       progressives" [syn: backing, backup, championship,
       patronage]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "backup":
   about-face, about-turn, advocate, agent, alter ego, alternate,
   alternative, amicus curiae, analogy, attorney, back track,
   back trail, backing, backing off, backing out, backing up,
   backup man, champion, change, changeling, comparison, copy,
   counterfeit, deputy, double, dummy, equal, equivalent, ersatz,
   exchange, executive officer, exponent, fake, figurehead, fill-in,
   ghost, ghostwriter, imitation, lieutenant, locum, locum tenens,
   makeshift, metaphor, metonymy, mock, next best thing, paranymph,
   personnel, phony, pinch, pinch hitter, pleader, procurator,
   provisional, proxy, relief, replacement, representative, reserve,
   reserves, reversal, reverse, reversing, reversion, right-about,
   right-about-face, ringer, second in command, second string,
   secondary, sign, spare, spares, stand-in, stopgap, sub,
   substituent, substitute, substitution, succedaneum, superseder,
   supplanter, surrogate, swingaround, symbol, synecdoche, temporary,
   tentative, third string, token, turnabout, turnaround, understudy,
   utility, utility man, utility player, vicar, vicar general,
   vicarious, vice, vice-president, vice-regent, vicegerent,
   volte-face



The Free On-line Dictionary of Computing (30 December 2018):

backup

    ("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 commonly refers to a copy of the files on a
   computer's disks, 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 backup software, differential backup,
   incremental backup, full backup.  Compare archive,
   source code management.

   (2004-03-16)