WordNet (r) 3.0 (2006):

uptime
    n 1: a period of time when something (as a machine or factory)
         is functioning and available for use [ant: downtime]

The Jargon File (version 4.4.7, 29 Dec 2003):

uptime
 n.

    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.