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.

[Jargon File]

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Head Normal Form
head normalisation theorem
heads down
heat sink
heat slug
heavy metal