Duplicating a system's critical components so the system keeps working if one fails. It can be active (both run at once) or passive (one is on standby).
The basis for high availability and fault tolerance across every cloud exam. In AZ-900: redundant storage (LRS, ZRS, GRS). In CLF-C02: S3 replication. In SAA-C03: Multi-AZ for RDS.
Redundancy is a concept that can show up on the exam. Check the full guide to see it in context.