watchdog timer

It is basically a maintenance chip that monitors the health of the CPU and can reset the CPU if it hangs up. Usually it will capture the contents of the important registers at the time of failure for troubleshooting.

See http://en.wikipedia.org/wiki/Watchdog_timer

In addition to what techno said, a watchdog timer can also be a simple timer relay that "watches" for the presence of pulses.

Say you have a conveyor that you want to monitor for belt breakage. You put a bolt on one of the rollers and put a proximity sensor on it so that it gives a pulse everytime the roller makes a complete revolution. If the pulses stop but the conveyor motor is still running, you can interpret that to mean that the belt has broken.

In PLCs, the internal watchdog monitors the scan time of the processor. If the scan time exceeds the watchdog setting, the processor will generate a fault and possibly stop processing.

it doesn't have to be an external circuit, PIC's for instance have an internal watchdog that can be set anywhere between a mS or several seconds.

unless the watchdog is cleared regularly then the chip will reset.

basically stops the processor getting stuck in a loop somewhere if the loop has no clear watchdog command.