A periodic 'I'm alive' signal a process emits so other systems can detect when it stops, the inverse of polling.
A heartbeat is a small, regular message a process or component emits on a fixed cadence (every 1-30s) so other systems know it's still alive. Common uses: agents reporting in to a control plane, leader candidates in a distributed consensus protocol, replicas signaling lag to a coordinator, on-call paging tools confirming the integration is wired. When heartbeats stop arriving, the receiver knows the source is gone, even if no error was returned.
The hardest failure to detect is silence, a process that hung but didn't crash, a network path that black-holed packets, a region that became unreachable but returned no error. Heartbeats turn that silent failure into a loud one: 'I haven't heard from this agent in 90 seconds, page someone.' Pair every long-lived integration with a heartbeat-based dead-man's-switch alert.
See the part of the platform that handles heartbeat in production.