Regiment, traditionally the largest military unit belonging to a single arm or branch of service, such as infantry or cavalry; the command of a colonel. Historically regiments were more permanent than other kinds of army units, each having its colors (battle flags), battle honors, and traditions. For these reasons the U.S. Army retains regiments as “parent” organizations of battalions and other units, although the regiment has ceased to be a tactical unit. British regiments have a similar function.

Historically, the strength of a regiment has varied from 600 to 4,000 men. The regiment has usually consisted of 8 to 12 line-of-battle, or line, companies, and certain specialist companies. The line-of-battle companies were originally called the battalion. Later regiments of 9 to 12 companies were divided into three equal battalions.

In the British army the regiment became almost entirely a traditional unit, headed by an honorary colonel of the royalty or nobility. It retained some administrative functions, but battalions became the tactical units with no definite number assigned to a regiment.