Uptime is a measure of the time a computer system has been up and running. It came into use to describe the opposite of downtime, times when a system was non-operational.
It is often used a measure of computer operating system reliability and stability, in that this time represents the time a computer can be left unattended without crashinging, or needing to be rebooted for administrative or maintenance purposes.
Netcraft maintains the uptime records for many thousands of web-hosting computers.