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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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