Uptime

Uptime is the share of a time window during which a service is up and responding correctly. It is usually expressed as a percentage — 99.9%, 99.99% — and measured by an external monitor that checks the service on a schedule.

Higher uptime targets allow surprisingly little downtime: 99.9% over a year is about 8 hours 45 minutes, and 99.99% is under an hour. How accurately you can claim a number depends on how often you measure it.