Uptime (availability)
Uptime measures the share of time a service responds correctly. Commitments are counted in nines: 99.9% tolerates roughly 8 hours 45 minutes of downtime per year; 99.99% allows less than one hour.
In practice
Honest measurement comes from external probes, across multiple regions, testing the real user path — not from a server declaring itself alive. Every minute of downtime has a direct cost and a deferred one: search engines space out their visits to unstable sites. Monitoring alerts before visitors become the detection system.