Disaster recovery (DR) is the set of policies, tooling, and infrastructure for restoring service after a major failure — a datacenter or whole-region outage, data corruption, or catastrophe — as opposed to routine failover of a single node. It is measured by how much data and time you can afford to lose (RPO and RTO).
Worked example: a company keeps replicas and backups in a second region; if the primary region goes dark, the DR plan promotes the standby region to serve traffic within its target recovery time. Gotcha: a DR plan you have never tested is not a plan — untested backups fail to restore, runbooks go stale, and dependencies get missed — which is why teams run periodic DR drills (game days) and why chaos engineering exists.