A person or thing that can be called on if necessary; a reserve. The team has a backup goalie in case the starter gets injured.
A copy of data stored separately from the original, used to restore the original in case of data loss. It's important to create a backup of your files to prevent data loss.
An accumulation of something caused by a blockage or delay. There was a traffic backup on the highway due to an accident.
Help or reinforcements, especially in law-enforcement, military, or emergency situations. The officer radioed for backup when the suspect fled.