in most armies, a body of troops headed by a colonel and organized for tactical control into companies, battalions, or squadrons. French cavalry units were called regiments as early as 1558. The word is derived from the Latin regimen, a rule or system of order, and describes the regiment's functions of raising, equipping, and training troops. As a regiment acquired individuality, colours, coat of arms, distinctive uniform and insignia, and achievements in battle, it also became a central object of loyalty, pride, and esprit de corps of its soldiers.

In early U.S. service, as in European armies up to that time, the usual number of companies in a regiment was 10. The armies of the French Revolution were reorganized into three-battalion “demibrigades” that were later renamed regiments. In 19th-century Europe, three-battalion regiments increasingly became the norm, though some of Napoleon (Napoleon I)'s regiments had as many as five with the colours. Later, Edward Cardwell (Cardwell, Edward Cardwell, Viscount) reorganized the British infantry into two-battalion regiments, each having one battalion at home and one stationed overseas. The U.S. Army adopted a three-battalion infantry regimental organization in 1901 and incorporated it into the divisions employed in World Wars I and II and in the Korean War.