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Period when a computer system is available
uninterrupted system availability

Uptime is a measure of system reliability, expressed as the percentage of time a machine, typically a computer, has been working and available. Uptime is the opposite of downtime. It is often used as a measure of computer operating system reliability or stability, in that this time represents the time a computer can be left unattended without crashing or needing to be rebooted for administrative or maintenance purposes. Conversely, long uptime may indicate negligence, because some critical updates can require reboots on some platforms.

In 2005, Novell reported a server with a 6 year uptime. Although that might sound unusual, that is actually common when servers are maintained under an industrial context and host critical applications such as banking systems. Netcraft maintains the uptime records for many thousands of web hosting computers. A server running Novell NetWare has been reported to have been shut down after 16 years of uptime due to a failing hard disk. A Cisco router has been reported to have been running continuously for 21 years as of 2018. As of April 11, 2023, the uptime has increased to 26 years, 25 weeks, 1 day, 1 hour, and 8 minutes until the router was later decommissioned and the final screenshot of the uptime was 26 years, 28 weeks, 2 seconds, and 6 minutes.

Some versions of Microsoft Windows include an uptime field in Windows Task Manager, under the "Performance" tab. The format is D:HH:MM:SS (days, hours, minutes, seconds).

The uptime command is also available for FreeDOS. The version was developed by M. Aitchison.