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.