Source : WordNet®

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 : Free On-Line Dictionary of Computing

heartbeat
     
        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 synchronisation 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}, {watchdog}.
     
        [{Jargon File}]
     
        (1996-03-12)