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 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. [Jargon File] (1996-03-12)
