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