A division is a large military unit or formation, usually consisting of between 6,000 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 they belong to being less important

Note that a similar word, divizion/divizijun/dywizjon, is also used in languages such as Russian, Serbian, Croatian and Polish, for a battalion-size artillery or cavalry unit. And in naval usage "division" has a completely different range of meanings. Aboard ship (including coastguard vessels), in shore commands and in naval aviation units (including 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.