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.