More precisely on computers it is uninterrupted uptime (the system can not be turned off, rebooted or crashed). It's a contiguous amount of time that the machine has been turned on and the operating system running normally. As soon as you reboot your uptime starts over at 0. I recently had one of my servers up for 490 days straight until an extended power outage (6 hours) and it has been up 157 days since the power came back on.
If you are running MacOS X I'll bet you have an uptime command just like ours on Linux that will give you your uptime.
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[ November 17, 2002: Message edited by: void main ]