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}.
[glossary]