A battalion is a military unit of around 500-1500 men usually consisting of between two and seven companies and typically commanded by a Lieutenant Colonel. Several battalions are grouped to form a regiment or a brigade.

The nomenclature varies by nationality and by branch of arms, for instance, some armies organize their infantry into battalions, but call battalion-sized cavalry, reconnaissance, or tank units a squadron or a regiment instead. There may even be subtle distinctions within a nation's branches of arms, such as a distinction between a tank battalion and an armored squadron, depending on how the unit's operational role is perceived to fit into the army's historical organization.

A battalion is generally the smallest military unit capable of independent operations (i.e. not attached to a higher command), although many armies have smaller units that are self-sustaining. The battalion is usually part of a regiment, group or a brigade, depending on the organizational model used by that service. The bulk of a battalion will ordinarily be homogeneous with respect to type (e.g. an infantry battalion or a tank battalion), although there are many exceptions. Every battalion will also include some sort of combat service support, typically organized within a combat support company.

The term is Italian in origin, appearing as battaglione. The French changed the spelling to bataillon, whereupon it directly entered into German.

British Army

An infantry battalion is numbered ordinally 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 (2i/c). 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 WWII had around 845 men in it. With successive rounds of cutbacks after the war, many infantry Regiments were reduced to a single battalion (others were amalgamated to form large Regiments which maintained multiple battalions, eg. the Royal Anglian Regiment).

Important figures in a battalion headquarters include:

• Commanding Officer (CO) (invariably a Lieutenant Colonel or even a full Colonel)
• Second-in-Command (2i/c) (usually a Major or a lieutenant Colonel if the battalion is commanded by a full Colonel)
• Adjutant (Captain or Major)
• Quartermaster (QM) (Captain or Major)
• Regimental Medical Officer (RMO) (Army Medical Services Captain or Major)
• Regimental Administrative Officer (RAO) (Adjutant General's Corps Captain or Major)
• Padre (Royal Army Chaplains Department Chaplain 4th or 3rd Class)
• Regimental Intelligence Officer (RIO) (Intelligence Corps Lieutenant or Captain)
• Regimental Signals Officer (RSO) (Royal Corps of Signals Lieutenant or Captain)
• Regimental Sergeant Major (RSM) (Warrant Officer Class 1)
• Regimental Quartermaster Sergeant (RQMS) (Warrant Officer Class 2)