a person or thing that supports or reinforces another.
a musician or singer or group of musicians or singers accompanying a soloist: a singer with a three-man backup that plays cello, bass, and guitar.
an overflow or accumulation due to stoppage, malfunctioning, etc.: a sewage backup; a backup of cars at the tollbooth.
a person, plan, device, etc., kept in reserve to serve as a substitute, if needed.
a copy or duplicate version, especially of a file, program, or entire computer system, retained for use in the event that the original is in some way rendered unusable.
a procedure to follow in such an event.
Bowling. a ball that curves in a direction corresponding to the bowling hand of the bowler.
(of a person, plan, device, etc.) held in reserve as a substitute if needed: a backup driver; a backup generator.
performing a secondary or supporting function: A drummer and guitarist are the singer's backup musicians.

1775–85, Americanism; noun use of verb phrase back up

back up, backup.

See login. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Word Origin & History

see back up.
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

backup definition

operating system
("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.
The Free On-line Dictionary of Computing, © Denis Howe 2010
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Example sentences
Geothermal can meet that, without any need for auxiliary storage or a backup system.
Advance planning and backup systems allow colleges to recover data.
Computer servers need costly extra layers of backup.
When the going gets tough, for instance, some dolphins call for backup.
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