back·up 
(băkŭp′)
(băkŭp′)n.
1.
a. A reserve or substitute.
b. Computers A copy of a program or file that is stored separately from the original.
2.
a. Support or backing.
b. Music A background accompaniment, as for a performer.
3. An overflow or accumulation caused by clogging or by a stoppage: a backup in the sink; a backup of traffic at the drawbridge.
adj.
Extra; standby: a backup pilot.