Stands for Shoot The Other Node in the Head, and refers to a mechanism whereby the ``other node'' is unconditionally powered off by a command sent to a remote power supply.
This is the big hammer approach to I/O fencing. It is most often used by quorate clusters, since once the cluster membership is categorically established, it's a simple matter to power off those nodes who are not current members. Stonith is much less appropriate to resource driven clusters, since they often don't have sufficient information to know that a node should be powered off.
The main disadvantage inherent in stonith devices is that the situation in a split brain situation caused by genuine communications path failure, then the communication path to the remote power supply used to implement stonith is also likely to be disrupted.