Split Brain Syndrome and I/O Fencing (RAC).

Split brain syndrome occurs when the instances in a RAC fails to connect or ping
to each other via the private interconnect, Although the servers are physically up
and running and the database instances on these servers is also running.

The individual nodes are running fine and can accept user connections and work
independently.

So, in a two node situation both the instances will think that the other
instance is down because of lack of connection.

The problem which could arise out of this situation is that the sane block
might get read, updated in these individual instances which cause data integrity
issues, because the block changed in one instance will not be locked
and could be overwritten by another instance.

So, when a node fails, the failed node is prevented from accessing all
the shared disk devices and groups. This methodology is called I/O Fencing,
Disk Fencing or Failure Fencing. 

In a RAC environment the node which first detects the other unreachable node
will evict it from the cluster to avoid data corruption.