heartbeat

heartbeat n.  1. The signal emitted by a Level 2 Ethernet
   transceiver at the end of every packet to show that the
   collision-detection circuit is still connected.  2. A periodic
   synchronization signal used by software or hardware, such as a bus
   clock or a periodic interrupt.  3. The `natural' oscillation
   frequency of a computer's clock crystal, before frequency division
   down to the machine's clock rate.  4. A signal emitted at regular
   intervals by software to demonstrate that it is still alive.
   Sometimes hardware is designed to reboot the machine if it stops
   hearing a heartbeat.  See also {breath-of-life packet}.