legio

Legio is the Latin word for a legion, the principal military unit of the Roman army from the middle Republic to late antiquity. A legion was a self-contained expeditionary force capable of operating in hostile territory. In the imperial period it typically fielded about 4,800 infantry, plus a small cavalry contingent and support personnel, for a total often around 5,000 to 6,000 men. The legion was divided into ten cohorts; the first cohort was the largest and most prestigious. Each cohort consisted of six centuries, commanded by a tribune or senior centurion. The aquila, the eagle standard, served as the legion’s emblem and rallying point in battle.

Originally organized as manipular units, later legions adopted a cohort-based structure that improved flexibility and marching speed. Equipment varied over time, but standard heavy infantry commonly wore helmet and body armor, wielded a gladius (short sword) and pilum (javelin), and fought behind a large shield (scutum). A legion also included engineers, artillery, and other support troops; cavalry was provided mainly by auxilia attached to the legion rather than by the legion proper in the empire.

Legions were commanded by a legatus legionis, usually a senator of praetorian rank, with several senior officers and many centurions. They fought across the empire, from Gaul and Britain to the East. Notable legions include Legio X Equestris (Caesar’s personal legion), Legio XII Fulminata, Legio XIII Gemina, and Legio IX Hispana. The fate of IX Hispana is debated; other legions were reorganized or remained in service for decades under imperial authority.

Beyond antiquity, the term legio survives in modern usage as a general term for large military formations and appears in various historical and fictional contexts. The Latin word legio is the source of many modern terms translated as “legion” and continues to be a standard reference in studies of Roman military history.