Systems resilience is the ability of a system to keep delivering essential functions when it is under stress, degraded, or partially failed. In cyber security, this means a network, application, or service can continue operating even if some components are attacked, misconfigured, overloaded, or taken offline. A resilient design does not assume every control will always work; it assumes failures will happen and plans for them.
Resilience matters because many real attacks aim to disrupt availability rather than steal data. Denial-of-service floods, ransomware, destructive malware, and dependency failures can all knock services out if the design is fragile. Defenses that improve resilience include redundancy, failover, backups, segmentation, load balancing, rate limiting, monitoring, and graceful degradation. In practice, a resilient system may lose noncritical features first while protecting core services, then recover quickly once the incident is contained. This makes resilience a key part of business continuity and incident response.