Decimation, according to my memory of Gibbon, was a punishment for cowardice or disobedience during battle as practiced by the Roman army.

The century was divided into groups of 10 men each. The centurion would place 9 white beans and 1 black bean in his helmet and the each man of the 10 would draw a bean. The unlucky recipient of the black bean would be beaten to death by the others. If they obeyed in this, the man who died was allowed to leave property and belongings to his family. If they disobeyed, all ten would be beaten to death by the remainder of the century, and all or their property and belongings would be confiscated by the state.

This is strictly from memory, faulty at best. My Decline and Fall of the Roman Empire has no index, and there's no way I'm going to go on a search right now. Corrections welcome.

Sarah