In the military of ancient Rome, decimation (from Latin decimatio 'removal of a tenth'[1]) was a form of military discipline in which every tenth man in a group was executed by members of his cohort. The discipline was used by senior commanders in the Roman army to punish units or large groups guilty of capital offences, such as cowardice, mutiny, desertion, and insubordination, and for pacification of rebellious legions. The procedure was an attempt to balance the need to punish serious offences with the realities of managing a large group of offenders.[2]
Polybius
was invoked but never defined (see the help page).