Cohort (military unit)

A cohort (from the Latin cohors, plural cohortes) was the standard tactical unit of a Roman legion and was composed of 480 soldiers. A Cohort is considered to be the equivalent of a modern military battalion. The Cohort unit replaced the manipular system following the reforms traditionally attributed to Gaius Marius in 107 BC. Until the middle of the first century AD, 10 cohorts (the first of which was larger for about 5600 men total) made up a Roman Legion.

Legionary cohort
During 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. The first cohort was now 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.

The other cohort consisted of approximately 480 men in six centuriae 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 to 80. 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).

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).

The legion at this time numbered about 5,400 men, including officers, engineers and usually a small unit of cavalry (equites legionis; 120 men and horses).