Definitions for uptimeˈʌpˌtaɪm
Random House Webster's College Dictionary
the time during which a machine is operating or an employee is working.
Origin of uptime:
a period of time when something (as a machine or factory) is functioning and available for use
The period of time a computer has functioned since last requiring a reboot.
Origin: up (adjective, computing) + time
The New Hacker's Dictionary
Technically, a machine's time since last reboot; jargonically, how long a hacker has gone without sleep. “What's your uptime?” “Oh, about 28 hours so far, but I think I can probably do another 12.” This is, of course, a reference to the uptime command and the pride with which most Unix types note how long their computers go without reboots. Uptime is a testament to the stability of the OS and the stamina of the hacker.