The Collaborative International Dictionary of English v.0.48:

heartbeat \heartbeat\ n.
   the audible and palpable rhythmic contraction and expansion
   of the arteries with each beat of the heart; as, he listened
   to her heartbeat with a stethoscope.

   Syn: pulse, pulsation, beat.
        [WordNet 1.5]

   in a heartbeat immediately.
      [PJC]

WordNet (r) 3.0 (2006):

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, heartbeat, beat]
    2: a very short time (as the time it takes the eye to blink or
       the heart to beat); "if I had the chance I'd do it in a
       flash" [syn: blink of an eye, flash, heartbeat,
       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"

The Free On-line Dictionary of Computing (19 January 2023):

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)


The Jargon File (version 4.4.7, 29 Dec 2003):

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.