heartbeat
     n 1: the rhythmic contraction and expansion of the arteries with
          each beat of the heart; "he could feel the beat of her
          heart" [syn: {pulse}, {pulsation}, {beat}]
     2: a very short time (as the time it takes the eye blink or the
        heart to beat); "if I had the chance I'd do it in a flash"
        [syn: {blink of an eye}, {flash}, {instant}, {jiffy}, {split
        second}, {trice}, {twinkling}, {wink}, {New York minute}]
     3: an animating or vital unifying force; "New York is the
        commercial heartbeat of America"
Source: WordNet® 2.0

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}.


Source: The Jargon File