Decimation (Latin: decimatio; decem = “ten”) was a form of military discipline used by senior commanders in the Roman Army to punish units or large groups guilty of capital offences such as mutiny or desertion. The procedure was a pragmatic attempt to balance the need to punish serious offences with the practicalities of dealing with a large group of offenders.
A cohort selected for punishment by decimation was divided into groups of ten; each group drew lots (Sortition), and the soldier on whom the lot fell was executed by his nine comrades, often by stoning or clubbing.
The Greek historian Polybius wrote, “Instead they find a solution for the situation which chooses by a lottery system sometimes five, sometimes eight, sometimes twenty of these men, always calculating the number in this group with reference to the whole unit of offenders so that this group forms one-tenth of all those guilty of cowardice.”
The Romans were not the last to use the method of discipline, either. During the Thirty Years’ War, Archduke Leopold Wilhelm of Austria decided that the Madlon regiment was to be decimated: “On the first day of the execution the regiment’s cords and battle banners were broken by the executioner. On the second day, the chosen men were hanged on the trees on the road from Rokycany to Litohlavy.”
Today, of course, the term is frequently used instead to really mean “annihilation”, even though that’s pretty much the opposite of what the word originally meant.