Decimation: punishment in the Roman army. Of every ten soldiers, one was executed.
Decimation was never a common punishment: it was too harsh and would no longer inspire terror if it were applied too often. Our sources only rarely refer to it, but every reader knew what was meant. After a very serious offense (e.g., mutiny or having panicked), the commander of the commander of a legion would take the decision, and an officer would go to the subunit that was to be punished. By lot, he chose one in ten men for capital punishment. The surviving nine men were ordered to club the man to death. The Greek historian Polybius of Megalopolis describes the procedure:
The tribune assembles the legion, and brings up those guilty of leaving the ranks, reproaches them sharply, and finally chooses by lots sometimes five, sometimes eight, sometimes twenty of the offenders, so adjusting the number thus chosen that they form as near as possible the tenth part of those guilty of cowardice. Those on whom the lot falls are bastinadoed mercilessly [...]; the rest receive rations of barley instead of wheat and are ordered to encamp outside the camp on an unprotected spot. As therefore the danger and dread of drawing the fatal lot affects all equally, as it is uncertain on whom it will fall; and as the public disgrace of receiving barley rations falls on all alike, this practice is that best calculated both the inspire fear and to correct the mischief.note[Polybius, World History, 6.38.2-4; tr. H. J. Edwards.]
Probably, decimation was not usual in Polybius' days. It is recorded for the fifth century BCE, and is called "an ancestral punishment" by the Greek-Roman author Dionysius of Halicarnassus, but there are only a few known cases. However, the Roman commander Crassus (the future triumvir), who was fighting against Spartacus in 71 BCE, is said to have revived the punishment, which had fallen into disuse.
It is mentioned again during the civil wars, but was hardly applied during the empire, although a couple of instances are known, like the punishment of the Third legion Augusta (in the year 18). The latest recorded case of decimation is during the reign of Diocletian. It may have disappeared under influence of Christianity.