From The Free On-line Dictionary of Computing (19 January 2023):
heartbeat
1. <networking> 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.
(1996-03-12)