A centurion ( ; ), also hecatontarch in Greek sources ( or, in Byzantine times, κένταρχος) was a professional officer of the Roman army after the Marian reforms of 107 BC. Most centurions commanded a century (centuria) of 80 men, but senior centurions commanded cohorts, or took senior staff roles in their legion.

Centurions took their title from the fact that they commanded a century. Centuries were so-called because they originally numbered roughly 100 men. Just after that they numbered 60 men each and were paired into maniples, one with greater authority. After the Marian reforms, however, the standard establishment was set at 80 men.

Role

In the Roman infantry, centurions initially commanded a centuria or "century" of 100 men, then 60 and finally 80. During the Imperial era Centurions gradually rose in seniority in their cohort, commanding centuries with higher precedence, until commanding the senior century and therefore the whole cohort. The very best centurions were then promoted to become centurions in the First Cohort, called Primi Ordines, commanding one of the ten centuries and also taking on a staff role. The most senior centurion of the legion was the 'Primus Pilus'' who commanded the first century.

All centurions, however senior, had their own allocated century.

The Primus Pilus was so called because his own century was the first file (primus pilus) of the first (rightmost) cohort. Only eight officers in a fully officered legion outranked the Primus Pilus: The legate (legatus legionis), commanding the legion; the senior tribune (tribunus laticlavus), second-in-command of the legion; the Camp Prefect (praefectus castrorum); and the five other tribunes (tribuni angusticlavii) who apparently served as senior staff officers to the legate with a rank roughly eqivalent to a modern colonel.

In comparison to a modern military organization, centurions covered a whole range of ranks. Ordinary century commanders would be equivalent to a modern army lieutenants or captains. Mid-level centurions commanding centuries are possibly roughly equivalent in grade level to modern majors with the senior centurions leading cohorts equivalent to lieutenant colonels. The Primus Pilus with his senior command (commanding the first cohort) and his staff role (as an advisor to the legate and his tribunes) might be considered equivalent to a modern colonel.

Centurions often suffered heavy casualties in battle, generally fighting alongside the legionaries they commanded. They usually led from the front, occupying a position at the front right of the century formation. They led and inspired their men by example. They also sought to display the skill and courage that got them to their rank in the first place. It is for these reasons that they often suffered a disproportionate number of casualties.