Uptime is the share of time a website stays online and reachable for visitors. It is usually written as a percentage — for example, “99.9% uptime” means the site is available almost all the time, with only tiny gaps.
The opposite is downtime: the moments when a site cannot be opened at all. Think of a shop’s opening hours — a shop that is almost never closed does far more business than one that keeps unexpectedly shutting its doors. For a website, downtime means lost visitors, lost sales and lost trust.
Good uptime depends heavily on reliable Hosting and a well-tuned Server, plus monitoring that alerts the owner the moment something goes wrong so it can be fixed quickly.