definition
A reserve or substitute.
example
If the goalkeeper is injured, we have a backup.
definition
A copy of a file or record, stored separately from the original, that can be used to recover the original if it is destroyed or damaged.
example
After the power failure, we had to restore the database from backup.
definition
An accumulation of material caused by a (partial) obstruction or (complete) blockage of the flow or movement of the material, or an accumulation of material that causes an overflow due to the flow being greater than the maximum possible flow.
example
The accident caused a mile-long backup on the highway.
definition
(law enforcement) reinforcements
example
He's got a gun! You'd better send for backup.