A militiaə/ is generally an army or some other fighting organization of non-professional soldiers, citizens of a nation, or subjects of a state, who can be called upon for military service during a time of need, as opposed to a professional force of regular, full-time military personnel, or historically, members of a warrior nobility class (e.g., knights or samurai). Generally unable to hold ground against regular forces, it is common for militias to be used for aiding regular troops by skirmishing, holding fortifications, or irregular warfare, instead of being used in offensive campaigns by themselves. Militia are often limited by local civilian laws to serve only in their home region, and to serve only for a limited time; this further reduces their use in long military campaigns.
With the emergence of professional forces (in the form of mercenaries whose livelihood was military service) during the Renaissance, Western European militias wilted; later however, they would be revived as part of Florentine civic humanism, which held that professional militaries were a result of corruption, and admired the Roman model. The civic humanist ideal of the militia was spread through Europe by the writings of Niccolò Machiavelli (According to Hörnqvist, The Prince, ch. 12 and 13, Discourses on Livy, and The Art of War.)
Beginning in the late 20th century, some militias (in particular officially recognized and sanctioned militias of a government) act as professional forces, while still being "part-time" or "on-call" organizations. For instance, the members of some U.S. Army National Guard units are considered professional soldiers, as they are trained to maintain the same standards as their "full-time" (active duty) counterparts.
Militias thus can be military or paramilitary, depending on the instance. Some of the contexts in which the term "militia" is used include:
Forces engaged in defense activity or service, to protect a community, its territory, property, and laws.
The entire able-bodied population of a community, town, county, or state, available to be called to arms.
A subset of these who may be legally penalized for failing to respond to a call-up.
A subset of these who actually respond to a call-up, regardless of legal obligation.
A private, non-government force, not necessarily directly supported or sanctioned by its government.
An irregular armed force enabling its leader to exercise military, economic, and political control over a subnational territory within a sovereign state (See: Warlord).
The word militia dates back to ancient Rome, and more recently to at least 1590 when it was recorded in a book by Sir John Smythe, Certain Discourses Military with the meanings: a military force; a body of soldiers and military affairs; a body of military discipline
The word Militia comes from ancient Latin, in which it meant defense service, as distinguished from a body of (armed) defenders which would be volgus militum. It should be noted that the term is used by several countries with the meaning of "defense activity" indicating it is taken directly from Latin.