A militia is generally an army or some other fighting organization of non-professional and/or part-time soldiers; citizens of a country, or subjects of a state, who may perform military service during a time of need, as opposed to a professional force of regular, full-time military personnel; or, historically, to members of a warrior-nobility class. When acting independently militias are generally unable to hold ground against regular forces; militias commonly support regular troops by skirmishing, holding fortifications, or conducting irregular warfare, instead of undertaking offensive campaigns by themselves. Local civilian laws often limit militias to serve only in their home region, and to serve only for a limited time; this further reduces their use in long military campaigns. Militias may also, however, serve as a pool of available manpower for regular forces to draw from, particularly in emergencies.

An army, ground force or land force is an armed force that fight primarily on land. In the broadest sense, it is the land-based military branch, service branch or armed service of a nation or country. It may also include aviation assets by possessing an army aviation component. Within a national military force, the word army may also mean a field army.