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A cohort (from the Latin cohors, plural cohortes) was the basic tactical unit of a Roman legion following the reforms of Gaius Marius in 107 BC.
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Immediately after the Marian reforms, a Roman legion comprised ten cohorts, known simply as "The first cohort", "The second cohort" etc. The first cohort was considered to be the most senior and prestigious, and the tenth the least.
A cohort consisted of six "centuries" or centuria of 80 men, each commanded by a centurion assisted by junior officers. At various times prior to the reforms, a century might have meant a unit of 60, 80 or 100 men. It is almost certain that the most senior centurion of the six would have commanded the entire cohort. In order of seniority, the six centurions were titled hastatus posterior, hastatus prior, princeps posterior, princeps prior, pilus posterior and pilus prior (most senior). This followed the order of seniority in the earlier legions, where the youngest and least experienced units were termed hastati, next principes, and the oldest and most experienced triarii (pilus was an alternative name for triarius, the singular of triarii).
During the first century AD, the command structure and make-up of the legions was formally laid down, in a form that would endure for centuries. The first cohort was now made up of five double-strength centuries totalling 800 men, the centurion of its first century automatically being the most senior in the legion: the primus pilus or "first file". (pilus meant file whilst pilum meant spear.)
The legion at this time numbered about 5,400 men, including officers, engineers and usually a small unit of cavalry (equites; 120 men and horses).[1]
Auxiliary cohorts could be quingenaria (nominally 500 strong) or milliaria (1000 strong).
Some paramilitary corps in Rome consisted of one or more cohorts, though none were part of a legion:
Furthermore, the Latin word cohors was used in a looser way to describe a rather large "company" of people (see, for instance, cohors amicorum).