- Standby, reserve or extra.
"I am only a backup player."
- (computing) That is intended as a backup.
"Make a backup copy of that file."
- (computing) 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.
"After the power failure, we had to restore the database from backup."
- A reserve or substitute.
"If the goalkeeper is injured, we have a backup."
- (law enforcement) reinforcements
"He's got a gun - you better send for backup."
- An accumulation of material that halts the flow or movement of something.
"The blockage caused a backup in the plumbing."