backup

noun

1. a person or thing that supports or reinforces another.
2. 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.
3. an overflow or accumulation due to stoppage, malfunctioning, etc.: a sewage backup; a backup of cars at the tollbooth.
4. a person, plan, device, etc., kept in reserve to serve as a substitute, if needed.
5. Computers.
   1. 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.
   2. a procedure to follow in such an event.
6. Bowling. a ball that curves in a direction corresponding to the bowling hand of the bowler.

adjective

7. (of a person, plan, device, etc.) held in reserve as a substitute if needed: a backup driver; a backup generator.
8. performing a secondary or supporting function: A drummer and guitarist are the singer’s backup musicians.

see back up.