A disaster recovery service that replicates VMs and physical workloads to a secondary Azure region. If the primary site fails, you can fail over to the secondary site with minimal downtime.
Azure's DR solution in AZ-900 and AZ-104. Distinguished from Azure Backup: Backup restores lost data, while Site Recovery keeps applications available during a disaster. It minimizes RTO.
Azure Site Recovery is a concept that can show up on the exam. Check the full guide to see it in context.