A division is a large military unit or formation, usually consisting of between 10,000 and 25,000 soldiers. In most armies, a division is composed of several regiments or brigades; in turn, several divisions typically make up a corps.

Historically, the division has been the default combined arms unit capable of independent operations. Smaller combined arms units, such as the American regimental combat team (RCT) during World War II, were used when conditions favored them. In recent times, modern Western militaries have begun adopting the smaller brigade combat team (similar to the RCT) as the default combined arms unit, with the division to which they belong being less important.

A similar word, divizion/divizijun/dywizjon, is also used in Slavic languages (such as Russian, Serbo-Croatian, and Polish) for a battalion-size artillery or cavalry unit. In naval usage "division" has a completely different range of meanings. Aboard ships of British Royal Navy tradition, the terms "division" or "department" are often used interchangeably and refer to the administrative organization used to manage personnel. Aboard US Navy ships (including US Coast Guard vessels), in shore commands and in US naval aviation units (including US navy, marine corps, and coastguard aviation) it refers to an administrative/functional sub-unit of a department (e.g., fire control division of the weapons department). Alternatively it refers to a sub-unit of several ships within a flotilla or squadron, or to two or three sections of aircraft operating under a designated division leader.

In 1577, the Devon Trained Bands of England were divided into three divisions, each with two colonels and six captains. Among Western military theorists, one of the first to think of organizing an army into smaller combined-arms units was Maurice de Saxe (d. 1750), Marshal General of France, in his book Mes Rêveries. He died at the age of 54, without having implemented his idea. Victor-François de Broglie put the ideas into practice. He conducted successful practical experiments of the divisional system in the Seven Years' War.

The first war in which the divisional system was used systematically was the French Revolutionary War. Lazare Carnot of the Committee of Public Safety, who was in charge of military affairs, came to the same conclusion about it as the previous royal government, and the army was organised into divisions.

It made the armies more flexible and easy to maneuver, and it also made the large French Revolutionary Army manageable. Under Napoleon, the divisions were grouped together into corps, because of their increasing size. Napoleon's military success spread the divisional and corps system all over Europe; by the end of the Napoleonic Wars, all armies in Europe had adopted it.

The composition of divisions varied significantly during the two world wars and, in addition, there was no uniformity of quality, even when divisions had the same composition.

The size of infantry divisions deployed by the major belligerents at start of the Great War ranged from about 16,000 in the French and Russian armies to 17,500 in the German imperial army and about 18,000 in Austro-Hungarian and British armies.