Quorum
When every heartbeat to a peer goes stale, a node cannot tell whether that peer died or is merely isolated from it. Both look identical from where it stands, and the two call for opposite reactions: take the services over, or leave them alone because the peer is still running them.
Quorum is the tie-breaker. Arbitrators, sitting outside the cluster interconnect, are asked who they can still see, and a node that finds itself in the smaller half stops rather than risk running a service its peer is also running.
Enabling quorum, sizing the arbitrators and testing them is covered in Cluster Quorum. The two examples below work through the arithmetic on real cluster shapes: