<p><strong>Noun</strong></p><ul><li>a person or thing that supports or reinforces another.</li><li>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.</li><li>an overflow or accumulation due to stoppage, malfunctioning, etc.: a sewage backup; a backup of cars at the tollbooth.</li><li>a person, plan, device, etc., kept in reserve to serve as a substitute, if needed.</li><li>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.</li><li>a procedure to follow in such an event.</li><li>Bowling: a ball that curves in a direction corresponding to the bowling hand of the bowler.</li></ul><p><strong>Adjective</strong></p><ul><li>(of a person, plan, device, etc.) held in reserve as a substitute if needed: a backup driver; a backup generator.</li><li>performing a secondary or supporting function: A drummer and guitarist are the singer's backup musicians.</li></ul>