Backup system that's fully running and ready to take over instantly if the primary system fails. Like having a second pilot ready to take the controls immediately.
Banking systems use hot standby databases that are always synchronized and can take over in milliseconds during failures.
All provide a standby replica that stays continuously synchronized with the primary so failover can happen quickly. AWS RDS Multi-AZ and Cloud SQL HA focus on managed database HA with automatic failover within a region; Azure Active Geo-Replication focuses on readable secondaries and failover across regions; OCI Data Guard provides physical/logical standby databases with fast switchover/failover.