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)