Irregular military is any military component distinct from a country's regular armed forces, representing non-standard militant elements outside of conventional governmental backing. Irregular elements can consist of militias, private armies, mercenaries, or other non-state actors, though no single definition exists beyond exclusion from national service. Without standard military unit organization, various more general names are often used; such organizations may be called a troop, group, unit, column, band, or force. Irregulars are soldiers or warriors that are members of these organizations, or are members of special military units that employ irregular military tactics. This also applies to irregular infantry and irregular cavalry units.

Irregular warfare is warfare employing the tactics commonly used by irregular military organizations. This often overlaps with asymmetrical warfare, avoiding large-scale combat and focusing on small, stealthy, hit-and-run engagements.

The words "regular" and "irregular" have been used to describe combat forces for hundreds of years, usually with little ambiguity. The requirements of a government's chain of command cause the regular army to be very well defined, and anybody fighting outside it, other than official paramilitary forces, are irregular. In case the legitimacy of the army or its opponents is questioned, some legal definitions have been created.^{[citation needed]}

In international humanitarian law, the term "irregular forces" refers to a category of combatants that consists of individuals forming part of the armed forces of a party to an armed conflict, international or domestic, but not belonging to that party's regular forces and operating inside or outside of their own territory, even if the territory is under occupation.

The Third Geneva Convention of 1949 uses "regular armed forces" as a critical distinction. The International Committee of the Red Cross (ICRC) is a non-governmental organization primarily responsible for and most closely associated with the drafting and successful completion of the Third Geneva Convention Relative to the Treatment of Prisoners of War ("GPW"). The ICRC provided commentary saying that "regular armed forces" satisfy four Hague Conventions (1899 and 1907) (Hague IV) conditions. In other words, "regular forces" must satisfy the following criteria:

By extension, combat forces that do not satisfy these criteria are termed "irregular forces".

The term "irregular military" describes the "how" and "what", but it is more common to focus on the "why" as just about all irregular units were created to provide a tactical advantage to an existing military, whether it was privateer forces harassing shipping lanes against assorted New World colonies on behalf of their European contractors, or Auxiliaries, levies, civilian and other standing irregular troops that are used as more expendable supplements to assist costly trained soldiers. Bypassing the legitimate military and taking up arms is an extreme measure. The motivation for doing so is often used as the basis of the primary label for any irregular military. Different terms come into and out of fashion, based on political and emotional associations that develop. Here is a list of such terms, which is organized more or less from oldest to latest:

Intense debates can build up over which term is to be used to refer to a specific group. Using one term over another can strongly imply strong support or opposition for the cause.