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.

