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)