The time since a computer was started is referred to as its uptime. When the computer is rebooted (or crashes), its uptime is reset to zero. The time the computer is not running is called downtime.

When running a service, such as a website, we want it to be available at all times. This means that the server the website is running on must be available all the time. As a high uptime means that a server has been running a long time without interruption, it can indicate that the provided services are also available with few interruptions.