Uptime is the share of time a system or service is operating and reachable by users, usually expressed as a percentage over a defined window such as a month. It addresses the reliability problem of keeping critical functionality consistently available despite failures, deployments, and dependency issues. At a high level, uptime is derived from monitoring signals like health checks and synthetic requests that confirm the service can be contacted and is responding correctly; when those checks fail for long enough to breach the threshold, the interval is counted as downtime and investigated.
With uptime measurement and targets, teams can set clear availability expectations, detect incidents quickly, and prioritize fixes that reduce repeat outages; without it, availability becomes subjective, outages are discovered later, and it is harder to learn from failures or meet service commitments. This gap exists because modern systems are composed of many interdependent components where small issues can cascade into user-visible unavailability.