jargon.artrocity.com
New Hacker's Dictionary the Jargon File v 4.4.6
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.