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. a.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.
b.a procedure to follow in such an event.
EXPAND6.Bowling. a ball that curves in a direction corresponding to the bowling hand of the bowler.
COLLAPSE