Split-brain is a failure mode where more than one node believes it is the leader and starts making authoritative decisions.

In a healthy leader-based system, one node coordinates writes and others follow. That single-leader rule is what gives the system one ordered history of changes.

Split-brain breaks the rule: a network partition, slow failure detector, or unsafe failover can leave the old leader still accepting writes while another node is promoted in its place. The system now has two sources of truth.

The damage is not always obvious at the moment. It appears later, when the system tries to merge two histories that should never have existed.