uptime

from 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}]

from Jargon File (4.4.4, 14 Aug 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.