Large, organized force armed and trained for war, especially on land. The term may be applied to a large unit organized for independent action or to a nation's or ruler's overall military organization for land warfare. The character and organization of armies have varied through history. At various times armies have been built around infantry soldiers or mounted warriors (e.g., cavalry) or men in machines, and have been made up of professionals or amateurs, of mercenaries fighting for pay or for plunder, or of patriots fighting for a cause. See alsoair force, conscription, guerrilla, military unit, militia, U.S. Army.