heartbeat
1. <networking> 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 synchronisation 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, watchdog.