Uptime is the share of time a system is up and serving requests correctly, expressed as a percentage over a period — for example 99.9% over a 30-day month.
It is the headline reliability metric most teams report and most SLAs are written around. Uptime is the inverse of downtime: 99.9% uptime means 0.1% downtime, or about 43 minutes a month.
Because a service can look “up” from one place while failing from another, trustworthy uptime figures come from multi-region checks rather than a single probe.
