The Roman army was called the Roman army (Latin: exercitus Romanus), which was the military force serving ancient Rome throughout its history from the Roman Kingdom through the Republic and Empire

. The main and most famous military unit within the Roman army was the legion (Latin: legio), a large, well-organized body of infantry composed of Roman citizens known as legionaries. Each legion typically consisted of about 5,000 to 6,000 soldiers

. The legion was subdivided into smaller units:

• The basic unit was the century , consisting of about 80 men.
• Ten centuries formed a cohort.
• Ten cohorts made up a legion, with the first cohort being the most prestigious and larger than the others

The legion was commanded by a legate, usually a senator, and included cavalry and specialized troops for support

. The Roman army was highly disciplined and professional, especially during the Imperial period, with soldiers serving long terms of service

. In summary:

• The entire military force was called the Roman army (exercitus Romanus).
• The primary combat unit was the legion (legio), consisting of thousands of soldiers organized into cohorts and centuries

This structure and terminology were central to Roman military organization for centuries.