A battalion is a military unit, typically consisting of up to 1,000 soldiers. A battalion is commanded by a lieutenant colonel and subdivided into several companies, each typically commanded by a major or a captain. The typical battalion is built from three operational companies, one weapons company and one headquarters company. In some countries, battalions are exclusively infantry, while in others, battalions are unit-level organizations. The word battalion has its origins in the Late Latin word battalion, which is derived from battalia, meaning "battle" or "combat." The term was used to describe a large group of soldiers ready for battle. Over time, its meaning evolved in military terminology.
The word "battalion" came into the English language in the 16th century from the French bataillon, meaning "battle squadron" (similar to the Italian battaglione meaning the same thing) and the Spanish batallón, derived from the Vulgar Latin noun battalia ("battle") and ultimately from the Classical Latin verb battuere ("to beat" or "to strike"). The first use of the word in English is attested in the 1580s.
Description
A battalion is composed of two or more primary mission companies, which are often of a common type (e.g., infantry, tank, or maintenance), although there are exceptions, such as combined arms battalions in the U.S. Army. In addition to the primary mission companies, a battalion typically includes a headquarters staff and combat service support, which may be combined into a headquarters and service company. A battalion may contain a combat support company. With all these components, a battalion is the smallest military unit capable of "limited independent operations".

The battalion must have a source of resupply to enable it to sustain operations for more than a few days. This is because a battalion's complement of ammunition, expendable weapons (e.g., hand grenades and disposable rocket launchers), water, rations, fuel, lubricants, replacement parts, batteries, and medical supplies normally consists of only what the battalion's soldiers and the battalion's vehicles can carry.
The commander's staff coordinates and plans operations. A battalion's subordinate companies and their platoons are dependent upon the battalion headquarters for command, control, communications and intelligence, and the battalion's service and support structure. The battalion is usually part of a regiment, group, or brigade, depending on the branch of service.
NATO
NATO defines a battalion as being "larger than a company, but smaller than a regiment" while "consisting of two or more company-, battery-, or troop-sized units along with a headquarters." The standard NATO symbol for a battalion represented by a pair of vertical lines above a framed unit icon. Member nations have specified the various names they will use for organisations of this size.

General history
In the 16th and 17th centuries, a battalion represented a mass of 3,000–4,000 men, was divided into 15–20 companies, and (since the superstition of the time required odd numbers) was built so as to have 59 ranks in depth and 51 men at the front; while a pikemen, or spearmen, placed in the middle, were surrounded by three ranks of musketeers, other musketeers were located in separate, small quadrangles at the corners of a battalion and were often used as skirmishers (see also Pike and shot). Russia had a gulyay-gorod for a while, an analogue of Pike and shot. The increased effectiveness of artillery fire led to the use of a shallower formation and to the division of these large masses into several battalions. Large cavalry regiments (of 10 squadrons) also began to be divided into 2 battalions.
British Army
The term battalion is used in the British Army Infantry and some corps including the Royal Electrical and Mechanical Engineers and Intelligence Corps. It was formerly used in the Royal Engineers (before they switched to regiments), and was also used in the now defunct Royal Army Ordnance Corps and Royal Pioneer Corps. Other corps usually use the term "regiment" instead.
An infantry battalion is numbered ordinarily within its regiment (e.g., 1st Battalion, The Rifles, usually referred to as 1 Rifles). It normally has a headquarters company, support company, and three rifle companies (usually, but not always, A, B and C companies). Each company is commanded by a major, the officer commanding (OC), with a captain or senior lieutenant as second-in-command (2IC). The HQ company contains signals, quartermaster, catering, intelligence, administration, pay, training, operations and medical elements. The support company usually contains anti-tank, machine gun, mortar, pioneer and reconnaissance platoons. Mechanised units usually have an attached light aid detachment (LAD) of the Royal Electrical and Mechanical Engineers (REME) to perform field repairs on vehicles and equipment. A British battalion in theatre during World War II had around 845 men; as of 2012, a British battalion had around 650 soldiers. With successive rounds of cutbacks after the war, many infantry regiments were reduced to a single battalion (others were amalgamated to form large regiments that maintained multiple battalions, e.g., the Royal Anglian Regiment).

Important figures in a battalion headquarters include:
Commanding officer (CO) (invariably a lieutenant colonel)
Second-in-command (2i/c) (major)

Adjutant (captain)
Quartermaster (QM) (LE major)
Quartermaster (technical) (QM(T)) (LE captain)

Medical officer (MO) (Royal Army Medical Service captain or major)
Administrative officer (Adjutant General's Corps captain or major)
Padre (Royal Army Chaplains Department chaplain 4th or 3rd class)
Operations officer (OpsO) (senior captain)
Intelligence officer (IO) (lieutenant or captain)
Regimental Signals officer (RSO) (Royal Corps of Signals captain or senior captain of relevant corps)
Regimental sergeant major (RSM) (warrant officer class 1)
Regimental quartermaster sergeant (RQMS) (warrant officer class 2)
Regimental quartermaster sergeant (technical) (RQMS(T)) (warrant officer class 2)
Battalions of other corps are given separate cardinal numbers within their corps (e.g., 101 Battalion REME).
Battle group
A battle group consists of an infantry battalion or armoured regiment with sub-units detached from other military units acting under the command of the battalion commander.
Canadian Army
In the Canadian Army, the battalion is the standard unit organisation for infantry and combat service support and each battalion is divided into one or more sub-units referred to as companies.
In the Canadian Forces, most battalions are reserve units of between 100 and 200 soldiers that include an operationally ready, field-deployable component of approximately a half-company apiece. The nine regular force infantry battalions each contain three or four rifle companies and one or two support companies. Canadian battalions are generally commanded by lieutenant-colonels, though smaller reserve battalions may be commanded by majors.
Those regiments consisting of more than one battalion are:
The Royal Canadian Regiment (three regular and one reserve battalions)
Princess Patricia's Canadian Light Infantry (three regular battalions)
Royal 22e Régiment (three regular and two reserve battalions)
The Royal Newfoundland Regiment (two reserve battalions)
Tactically, the Canadian battalion forms the core of the infantry battle group, which also includes various supporting elements such as armour, artillery, combat engineers and combat service support. An infantry battle group will typically be commanded by the commander of the core infantry battalion around which it is formed and can range in size from 300 to 1,500 or more soldiers, depending on the nature of the mission assigned.
Indian Army
A battalion in the Indian Army consists of four rifle companies. In turn, each rifle company consists three platoons. A battalion in the Indian Army is commanded by a colonel. Normally a battalion is attached to a regiment of infantry, which is organised, as a general rule, of a number of battalions and the regimental centre battalion.
Royal Netherlands Army
In the Royal Netherlands Army, a mechanised infantry battalion usually consists of one command- and medical company, three mechanised infantry companies and one support company, which has three platoons with heavy mortars and three platoons with anti-tank missiles (TOW). With the Dutch artillery units, the equivalent of a battalion is called an afdeling (which translates to "section").
Combat companies consist of (usually mechanised) infantry, combat engineers, or tanks. In the latter case, the unit is called an eskadron, which translates roughly to "squadron". There are also support battalions in the Dutch Army, which specialise on a specific task: for example, supplies and transport or communications.
The Netherlands have four battalions that are permanently reserved for the United Nations, for the purpose of peacekeeping duties.
An infantry battalion, logistical battalion, combat battalion and the Netherlands Marine Corps all have a battalion structure. Each battalion usually consists of the following:
Battalion command
Commander
Second in command
General service
Personnel section
Intelligence section
Operations section
Materiel section
Communication section
Command company
Command group
Administration group
Medical group