A cohort was a standard tactical military unit of a Roman legion, though the standard changed with time and situation, and was composed of 480 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. Shortly after the military reforms of Marius, and until the middle of the third century AD, 10 cohorts made up a Roman legion. The cohorts were named "first cohort", "second cohort" etc. The first cohort gathered the most experienced legionaries, while the legionaries in the tenth cohort were the least experienced.
Legionary cohort
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 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. The first centurion of the first cohort was called primus pilus. 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. The centurion of its first century automatically was the most senior in the legion and was known as the primus pilus. The primus pilus could be promoted to praefectus castrorum,. The praefectus castrorum was 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. The reformed legion numbered about 5,000 men, including officers, engineers and usually a small unit of cavalry.
Some paramilitary corps in Rome consisted of one or more cohorts, though none were part of a legion:
The nine cohortes praetoriae, never grouped to a legion, the infamous Praetorians. The term was first used to refer tothe bodyguard of a general during the republic; later, a unit of imperial guards.
*Cohors togata was a unit of the Praetorian guard in civilian dress tasked with duties within the pomerium.
Cohortes urbanae, "urban cohort": military police unit patrolling in the capital.
Cohortes vigilum, "watchmen": unit of the police force which also was the fire brigade in the capital.
Cohors Germanorum : the unit of Germani custodes corporis.
Furthermore, the Latin word cohors was used in a looser way to describe a rather large "company" of people.
