Uptime is the percentage of time that a system, service, or application is operational and available for use. High uptime (often expressed as 'five nines' or 99.999%) indicates reliable service with minimal outages. System administrators track uptime as a key performance indicator. Service level agreements (SLAs) often specify minimum uptime requirements. Downtime—when systems are unavailable—can result from hardware failures, software problems, maintenance, or external factors. Planned maintenance downtime is scheduled during low-usage periods. Unplanned downtime from failures is more disruptive. Government digital services aim for high uptime to ensure citizens can access services when needed. Redundant systems, backup power, and disaster recovery planning improve uptime reliability.