What does "split-brain" mean?

What does "split-brain" mean?
"Split brain" is a condition whereby two or more computers or groups of computers lose contact with one another but still act as if the cluster were intact. This is like having two governments trying to rule the same country. If multiple computers are allowed to write to the same file system without knowledge of what the other nodes are doing, it will quickly lead to data corruption and other serious problems.
Split-brain is prevented by enforcing quorum rules (which say that no group of nodes may operate unless they are in contact with a majority of all nodes) and fencing (which makes sure nodes outside of the quorum are prevented from interfering with the cluster).