What is Uptime?

Legal Definition
Uptime is a measure of the 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.
-- Wikipedia
Legal Definition
Operational time of equipment where it is fully operational. Opposite of downtime.