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.

Computers .

a copy or duplicate version, esp. 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.

noun

alternate , extra , substitute , help , second , second string , spare , standby , supporting , understudy

adjective

emergency , reserve , secondary , standby , supplemental , supplementary