Reliability
The property of a system to continue providing its intended function correctly and predictably for a specified period under defined conditions.
Reliability is broader than an uptime percentage. Correctness, data integrity, recovery behavior, latency bounds, and dependency failure modes determine whether a service remains genuinely usable. A process can be running while producing wrong results or repeatedly missing its deadlines.
No single metric captures the property. Failure rate, error budgets, SLI/SLO measurements, recovery time, and—where relevant—data-loss bounds should be considered together. Redundancy can help, but common-cause failure, incorrect failover, or tightly coupled replication can introduce new failure modes.
Related: Service Level Indicator, Service Level Objective, Error Budget, Circuit Breaker, Retry.