n. 1. anything kept in reserve to serve as a substitute in case of failure or unavailability of the normal or primary object; -- used for devices, plans, people, etc. Also used attributively; as, there was no backup for the electrical supply; a backup motor; a backup generator. [ PJC ]

2. (Music) a musician or group of musicians accompanying a soloist, whether vocalists or instrumentalists.
Syn. -- accompaniment. [ PJC ]

3. a person or group of persons serving as reinforcement for another or others; as, the policeman called for backup when he was fired on. [ PJC ]

4. an accumulation, overflow, or reverse flow (in traffic or a liquid flow system) caused by a stoppage or other malfunction. [ PJC ]

5. (Computers) A copy of a program or data from a computer onto a data-storage medium, usually one that may be removed to safe storage at a distance from the computer; it is used to preserve data for use in the contingency that the original data on the computer may be lost or become unusable. A backup that is removed from the building housing the computer system (to provide protection against loss of data in a disastrous event such as a fire) is called off-site backup. Also used attributively; as, backup copy. [ PJC ]

6. (Computers) The act or process of creating a backup{ 5 }; as, they performed a full system backup every weekend. [ PJC ]