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.
Hot standby is an architecture pattern (not a single cloud service): a fully running secondary environment continuously kept in sync with the primary so it can take over with minimal downtime. Cloud providers implement it using combinations of load balancers, health checks, multi-zone/region deployments, and data replication (e.g., database replicas, cross-region replication, active-passive failover).