A regiment is a military unit, larger than a company and smaller than a division. Depending on mission, country of origin, and makeup, a modern regiment is similar to a brigade in size in that both range from a few hundred soldiers up to 2,000-3,000, depending on branch of service and method of organization. The modern unit varies in size, scope, administrative role from nation to nation, and within the armed forces of some nations (See and contrast: US Marine regiments vs. US Army Infantry, as well as the differing use by the US Army Cavalry.)
The term came into use in Europe around the end of the 16th century, when armies evolved from a collection of retinues following knights to a more formally organized structure.
The number of soldiers in a regiment fluctuates, generally depending on casualties and the manpower of the associated army. At its creation, for example, the typical Civil War-era American infantry regiment numbered around 600 (although heavy artillery regiments serving as infantry numbered upwards of 1,000 troops, a brigade-sized formation). Veteran federal regiments commonly experienced a steady decline in strength as the federal (unlike the Confederate) practice was to organize new regiments rather than rebuild old units. However, at the end of the war, Confederate regiments sometimes had less than 100 troops (barely company-sized).
During the post-Civil War years, American regiments rarely served as intact units. The practice was to scatter companies throughout western posts and forts. Company strength for a 10 company infantry regiment or a 12 company (troop after 1882) cavalry regiment hovered around 50 men, well below authorized levels.
In the British Army, for most purposes, the regiment is the largest "permanent" organisational unit. Above regimental level, organisation is changed to meet the tasks at hand. Because of their permanent nature, many regiments have long histories, often going back for centuries; the oldest British regiment still in existence is the Honourable Artillery Company, established in 1537, while the Royal Scots, formed in 1633, is the oldest infantry regiment. (These claims are contested on various points of precedence; see FAQ: Regiments, in general and especially: FAQ: Oldest Regiment in the British Army.)
The United States Army was also once organized into regiments, but presently uses the brigade instead, except for cavalry. Although every battalion or squadron is associated with a regiment for historical purposes, the only combat regiments are cavalry regiments which are attached to a corps. These regiments, who are associated generally for historical purposes, can be known as "parent regiments".
The United States Marine Corps continues to use the regimental system once also employed by the US Army in which a regiment is a permanent organization consisting of three battalions and all necessary support units to operate independently.
Types of regiment in Commonwealth armies
In the British Army and other armies modelled on it, such as Canada's and Australia's, the term "regiment" is used confusingly in two different ways: it can mean a ceremonial grouping or a tactical unit. Ceremonial regiments are not part of the army's day-to-day operational command structure, but regimental ties are maintained by the voluntary efforts of their members. In addition to combat units, other organizations are considered part of the ceremonial regimental family: regimental associations (retirees), bands and associated cadet groups. The parts of a ceremonial regiment have in common such things as a colonel-in-chief (usually a member of the royal family), battle honours (honours earned by one unit of a ceremonial regiment are shared by the whole regiment), ceremonial uniforms, cap badge, peculiarities of insignia, stable belt, and regimental songs.
Ceremonial armoured regiments are composed of one (usual) or more tactical regiments. For example, the two tactical regiments Le 12e Régiment blindé du Canada and Le 12e Régiment blindé du Canada (Milice) are both part of the ceremonial regiment Le 12e Régiment blindé du Canada. The only ceremonial armoured regiment of the British Army that consists of more than one tactical regiment is the Royal Tank Regiment, which currently has two (1 and 2 RTR), and once had many more.
All the country's artillery is considered part of a single ceremonial regiment. However, there are several tactical artillery regiments. They are designated by numbers, names or both. For example, the tactical regiments 1st Regiment, Royal Canadian Horse Artillery , 10th Field Artillery Regiment, RCA and many others are part of the ceremonial regiment The Royal Regiment of Canadian Artillery. In Britain, the Royal Regiment of Artillery works in the same way.
Ceremonial infantry regiments are composed of one or more battalions. When a regiment has only one battalion, the battalion may have exactly the same name as the regiment. This means that a battalion's name often contains the word "regiment" despite the fact that it is not a regiment. For example, The North Saskatchewan Regiment is the only battalion in the ceremonial regiment of the same name. When there is more than one battalion, they are distinguished by numbers, subsidiary titles or both. In Britain, every infantry battalion bears a number, even if it is the only remaining battalion in the regiment (in which case it is the 1st Battalion). Until after the Second World War, every regiment had at least two battalions. Traditionally, the regular battalions were the 1st and 2nd Battalions, the militia battalion was the 3rd Battalion, and the Territorial Army battalions were the 4th Battalion and up. A few regiments had up to four regular battalions and more than one militia battalion, which skewed the numbering, but this was rare. For this reason, although the regular battalion today (if there is only one) will always be the 1st Battalion, the TA battalions may have non-consecutive numbers.
The British Army also has battalion-sized tactical regiments of the Royal Engineers, Royal Corps of Signals, Army Air Corps, Royal Logistic Corps, Royal Military Police, and formerly of the Royal Corps of Transport.