Irregular military refers to any non-standard military. Being defined by exclusion, there is a lot of variance in what comes under the term. It can refer to the type of military organization, or to the type of tactics used.

An irregular military organization is a military organization which is not part of the standard regular army organization that is the state's official military force. Without standard military unit organization, various more general names are used; such organizations may also 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 troops, irregular infantry and irregular cavalry.

Irregular warfare is warfare employing the tactics commonly used by irregular military organizations. This involves avoiding large-scale combats, and focusing on small, stealthy, hit and run engagements.

The term "irregular military" describes the "how" and "what", but it's more common to focus on the "why". 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 in and out of fashion, based on political and emotional associations that develop. Here is a list of such terms, organized more-or-less oldest to latest.

• Militia -- civilians who are part-time soldiers
• Mercenary -- soldier for hire
• Rebel -- someone part of a rebellion or revolt, whether military or not
• Revolutionary -- someone part of a revolution, whether military or not
• Partisan -- In the 20th century, someone part of a resistance movement; especially in the World War II era. In the 18th and 19th century, a detachment or organization using irregular tactics as part of a conventional force.
• Commando -- soldier trained specifically in irregular military tactics, even if part of the regular army
• Rangers -- as a military term, similar to commandos; mostly US use.
• Paramilitary -- non-regular military with a claim to official status
• Guerrilla -- someone who uses low-level irregular military tactics
• Resistance fighter -- someone who is part of a resistance movement
• Freedom fighter -- irregular military motivated by higher goals -- very subjective
• Terrorist -- irregular military who target civilians; this term is almost always used pejoratively.
• Insurgent -- an alternate term for many of the above.

Intense debates can build up over which of these terms to use when referring to a specific group. Using one term over another can imply either strong support or opposition for the cause being fought over.

It is possible for a military to cross the line between regular and irregular. Isolated regular army units forced to operate without regular support for long periods of time can degrade into irregulars. As an irregular military becomes more successful, they may transition away from irregular, even to the point of becoming the new regular army if they win.

While the morale, training and equipment of the individual irregular soldier can vary from very poor to excellent, irregulars are usually lacking the higher-level organizational training and equipment that is part of regular army. This usually makes irregulars poor at what regular armies focus on — main-line combat. Other things being equal, major battles between regulars and irregulars heavily favor the regulars.

However, irregulars can excel at many other combat duties besides main-line combat, such as scouting, skirmishing, harassing, pursuing, rear-guard actions, cutting supply, sabotage, raids, ambushes and underground resistance. Experienced irregulars often surpass the regular army in these functions. By avoiding formal battles, irregulars have sometimes harassed high quality armies to destruction, as in the Battle of Carrhae.

Irregulars have a reputation for ruthlessness. Being outside the official army, they often don't see themselves bound by the laws of war. Beyond official supply lines, irregulars often supply themselves by confiscating civilian goods without compensation; this can be seen as pillaging. Operating without official support equipment, prisoners taken by irregulars might be killed when transportation isn't feasible; this can be seen as an atrocity. Over time, unrestrained irregulars can devolve into common bandits or roving death squads.

The total effect of irregulars is often underestimated. Since the military actions of irregulars are often small and unofficial, they are underreported or even overlooked. Even when engaged by regular armies, some military histories exclude all irregulars when counting friendly troops, but include irregulars in the count of enemy troops, making the odds seem much worse than they were. This may be accidental; counts of friendly troops often came from official regular army rolls that exclude unofficial forces, while enemy strength often came from visual estimates, where the distinction between regular and irregular were lost. If irregular forces overwhelm regulars, records of the defeat are often lost in the resulting chaos.

Use of large irregular forces featured heavily in wars such as the American Revolution, the Franco-Prussian War, the Russian Civil War, the Boer War, and especially the Eastern Front of World War II where hundreds of thousands of partisans fought on both sides. Irregular forces are usually most noted for their role in the Vietnam War. The use of irregular forces was also present in the Philippine American War.

Ongoing conflicts in Africa such as the First and Second Congo Wars are fought almost entirely by irregular forces.

In the dawn of civilization, all military forces were irregular. Regular armies grew slowly from personal bodyguards or elite militia. In Ancient warfare, most civilized nations relied heavily on irregulars to augment their small regular army. Even in advanced civilizations, the irregulars commonly outnumbered the regular army. Sometimes entire tribal armies of irregulars were brought in from internal native or neighboring cultures, especially ones that still had an active hunting tradition to provide the basic training of irregulars. The regulars would only provide the core military in the major battles; irregulars would provide all other combat duties. Notable examples of regulars relying on irregulars include auxiliary legions of Germanic tribes in the Roman Empire, Cossack regiments in Imperial Russia, and Native American forces in the far west of the Confederate States of America.

One could attribute the disastrous defeat of the Romans at the Battle of the Teutoburg Forest to the lack of supporting irregular forces; only a few squadrons of irregular light cavalry accompanied the invasion of Germany when normally the number of foederati legions would equal the regular legions; however, since irregulars won that battle, there are few surviving details. During the decline of the Roman Empire, irregulars made up an ever increasing proportion of the Roman military. At the end, there was little difference between the Roman military and the barbarians across the borders. Throughout history, most civilizations eventually fell to "barbarians", that is, irregular military forces, with minimal historical details.

As the spread of industrialism dried up the traditional source of irregulars, nations were forced take over the duties of the irregulars using specially trained regular army units. Examples are the light infantry in the British Army. By the modern age, all regular military are trained to also perform the actions previously left to irregulars.

• Velites — Irregular infantry in the army of the Roman Republic
• Minutemen — American irregular troops during the American Revolution
• Bashi-bazouk — Irregular mounted mercenary in the Ottoman Empire
• Franc-tireur — French irregular forces during the Franco-Prussian War
• Pindari — 18th century irregular horsemen in India
• Impi — Zulu military that also employed irregular tactics
• Asymmetric warfare — Military theory that also includes regulars vs. irregulars
• List of guerrillas

• Flavius Vegetius Renatus, Epitoma rei militaris
• Dr. Thomas M. Huber, Compound Warfare: An Anthology
• Clifford J. Rogers, Military Technical Revolution debate among historians
• John M. Gates, US Army & Irregular Warfare
• Harold P. Ford, CIA and the Vietnam Policymakers: Three Episodes 1962-1968
• Robert R. Mackey, "The UnCivil War: Irregular Warfare in the Upper South, 1861-1865," University of Oklahoma Press, 2004, ISBN 0806136243