Definition
Uptime is the percentage of time a site or service is reachable and functioning correctly over a given period, and it is the headline reliability metric in web hosting. It is usually expressed as a "number of nines": 99.9% (three nines) allows roughly 8.7 hours of downtime a year, 99.99% (four nines) allows under an hour, and 99.999% (five nines) allows under six minutes. Most reputable hosts publish an uptime guarantee in their SLA, typically 99.9% on shared plans and 99.99% on managed or enterprise plans, with service credits rather than refunds as the usual remedy. That contractual number is a floor, not a measurement: independent third-party monitors such as UptimeRobot, StatusCake and Pingdom poll the site from outside the host's own network at regular intervals, which is more useful than any figure the host reports itself. Higher nines cost more and matter more for revenue-critical sites than for a personal blog.
How it works
Uptime is measured by polling a site from multiple external locations at regular intervals (often every minute) and recording the percentage of successful responses. Downtime can come from anywhere in the chain: origin server failure, DNS issues, CDN outages, network problems, or even certificate expiry.
Why it matters
Every minute of downtime costs revenue, search ranking, and trust. The host's SLA is the contractual floor; what actually matters is real measured uptime over months. For e-commerce, a five-nines target is justified; for a blog, three nines is usually enough.