The button on a computer that resets it. I guess it works by briefly interrupting the power supply to some vital part of or component on the mainboard.
The reset button on most PC's is on the front of the case, next to the power switch or turbo button. I used to accidently hit the one on my computer all the time because a) I'm clumsy so whenever I crossed my legs or stretched out I ran the risk of resetting; and b) my computer is under my desk so I have this habit of trying to put in disks and CD's by touch, without looking.
I was annoyed with this -- it really messes with your uptime, ya know? -- so I unhooked the reset button from the motherboard and hooked it up to the internal PC speaker which is shrill and tinny anyhow so I could turn the speaker off (usually) and on (occasionally) whenver I wanted. No, it's not a brilliant hack, but it does turn two frustrating components into one useful one.