1. a copy of computer data stored separately for protection
2. help or support, especially in an emergency
3. a person or thing that serves as a substitute

1. serving as a substitute or reserve

Examples

• Make sure to create a backup of your important files.
• The police called for backup when the situation escalated.
• She works as a backup singer for the band.
• There’s a backup in the kitchen sink drain.
• We have a backup generator in case the power goes out.
• The backup quarterback entered the game in the fourth quarter.
• Traffic backup extended for miles on the highway.