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Centuria (Latin plural centuriae) is a Latin term (from the stem centum meaning one hundred) denoting military units consisting of (originally) 100 men. The size of the century changed over time and from the first century B.C.E. throughout most of the empire the standard size of a centuria was 80 men.
A centuria is also a Roman unit of land area corresponding to 100 heredia.
In the political context the centuria was the constituent voting unit in the assembly of the centuries (Latin comitia centuriata), an old form of popular assembly in the Roman Republic, the members of which cast one collective vote.
The centuria was the pivotal tactical Roman legion unit after the Marian reforms of 107 BC. It originally consisted of a hundred soldiers; later 80 distributed among 10 contubernia, with support staff making up the remainder of the 100 men. Each contubernium (the minimal unit in the Roman legion) consisted of 8 soldiers who lived in the same tent while on campaign or the same bunk room in barracks. Centuriae were grouped by pairs forming maniples, and later into 6 century cohorts.
A century was commanded by a Centurion, who was assisted by an Optio (lieutenant) and Tesserarius (sergeant). It had a banner or signum which was carried by a Signifer. Also, each century provided a Buccinator, who played a buccina, a kind of horn used to transmit acoustic orders.
On the battlefield, the Centurion stood at the far right of the first row of men next to the Signifer, while the Optio stood at the rear, to avoid, if necessary, the disbanding of the troops and ensure the relay between typical closed order lines used by the Roman army.
In the imperial period, but likely not the republican period, the first cohort (consisting of the bravest men from the legion) was twice the size of the other cohorts. Each of its five centuriae was a double centuria of 160 men (rather than 80). The first cohort thus consisted of 800 men. Centurions of these first-cohort double centuriae were called primi ordinis ("first rank"), except for the leader of the first centuria of the first cohort, who was referred to as primus pilus (first file).
The term centuria was later used during the Spanish Civil War to describe the informal bands of local militiamen and international volunteers that sprang up in Catalonia and Aragon in October-November 1936.