A centurion was an experienced professional officer who commanded a century in the Roman army. Possessing fighting skills and hardened in the battlefield, he was responsible of leading his men into the gory melee of hand to hand combat. A centurion was the most reliable soldier; a tough breed a military tribune could always count on. Most centurions commanded a century of 80 men, but senior centurions commanded cohorts, or took senior staff roles in their legion. The name “centurion” derives from the fact that they commanded a century. Centuries were so-called because they originally numbered roughly 100 men. But after the Marian military reforms of 107 BC, the standard establishment was set at 80 men.
During the Imperial era centurions gradually rose in seniority in their cohort, commanding centuries with higher precedence, and later 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.
The Primus Pilus centurion was so called because his own century was the primus pilus, which means the first file, or line, of the first rightmost cohort. Only eight officers in a fully officered legion outranked the Primus Pilus: The legate, commanding the legion; the senior tribune; the Camp Prefect; and the six tribunes who had a greater rank in the praetorian guard as they commanded a whole cohort.