A cohort (from the Latin cohors, plural cohortes) was a standard tactical military unit of a Roman legion and was composed of roughly 500 soldiers. A cohort is considered to be the equivalent of a modern military battalion. The cohort replaced the maniple following the reforms attributed to Gaius Marius in 107 BC. Until the middle of the third century AD, 10 cohorts (about 5000 men total) made up a Roman legion.
Originally, a cohort consisted of six centuriae, each commanded by a centurion assisted by junior officers. At various times prior to the reforms, a century might have 60 to 100 men. The cohort had no permanent commander; it is assumed that in combat, 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).
During the reforms in the 1st century AD, the command structure and make-up of the legions was formally laid down, in a form that would endure for centuries. Standard centuriae consisted of 80 men each. The first cohort was made up of five double-strength centuries totalling 800 men, the centurion of its 1st century automatically being the most senior in the legion. This century was known as the primus pilii (first files), and its centurion was known as the primus pilus (first file or first spear). The Primus Pilus could be promoted to Praefectus Castrorum, or "Camp Prefect." The Praefectus Castrorum would be in charge of the daily running of a legion.
These ranks followed the order of seniority in the earlier manipular legions, where the youngest and least experienced units were termed hastati, next principes, and the oldest and most experienced triarii (pilus was a rare alternative name for triarius, the singular of triarii).
Auxiliary cohorts could be quinquagenaria (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).
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