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"

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "heartbeat":
   anima, animating force, arrhythmia, atman, bathmism, beat, beating,
   beating heart, biological clock, biorhythm, blood, breath,
   breath of life, divine breath, divine spark, drumming, elan vital,
   essence of life, flutter, force of life, growth force, heart,
   heartblood, heartthrob, impulse of life, inspiriting force, jiva,
   jivatma, life breath, life cycle, life essence, life force,
   life principle, life process, lifeblood, living force, palpitation,
   pitapat, pitter-patter, pneuma, prana, pulsation, pulse, rat-a-tat,
   rataplan, rhythm, seat of life, soul, spark of life, spirit,
   staccato, tempo, throb, throbbing, vis vitae, vis vitalis,
   vital energy, vital flame, vital fluid, vital force,
   vital principle, vital spark, vital spirit



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.


The Free On-line Dictionary of Computing (30 December 2018):

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)