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, 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.