A centurion (; Latin: centurio [kɛn̪ˈt̪ʊrioː], pl. centuriones; Greek: κεντυρίων, translit. kentyríōn, or Greek: ἑκατόνταρχος, translit. hekatóntarkhos) was a position in the Roman army during classical antiquity, nominally the commander of a century (Latin: centuria), a military unit originally consisting of 100 legionaries. The size of the century changed over time, and from the first century BC through most of the imperial era was reduced to 80 men. In a Roman legion, centuries were grouped into cohorts and commanded by their senior-most centurion. The prestigious first cohort was led by the primus pilus, the most senior centurion in the legion and its fourth-in-command who was next in line for promotion to Praefectus Castrorum, and the primi ordines who were the centurions of the first cohort. A centurion's symbol of office was the vine staff, with which they disciplined even Roman citizens, who were otherwise legally protected from corporal punishment by the Porcian Laws. Centurions also served in the Roman navy. After the 107 BC Marian reforms of Gaius Marius, centurions were professional officers. In Late Antiquity and the Middle Ages, the Byzantine army's centurions were also known by the name kentarch (Kentarches).

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