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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.
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