Guerrilla warfare is a form of irregular warfare

Irregular warfare

Irregular warfare is warfare in which one or more combatants are irregular military rather than regular forces. Guerrilla warfare is a form of irregular warfare, and so is asymmetric warfare....

 and refers to conflicts in which a small group of combatant

Combatant

A combatant is someone who takes a direct part in the hostilities of an armed conflict. If a combatant follows the law of war, then they are considered a privileged combatant, and upon capture they qualify as a prisoner of war under the Third Geneva Convention...

s including, but not limited to, armed civilians (or "irregulars") use military tactics

Military tactics

Military tactics, the science and art of organizing an army or an air force, are the techniques for using weapons or military units in combination for engaging and defeating an enemy in battle. Changes in philosophy and technology over time have been reflected in changes to military tactics. In...

, such as ambush

Ambush

An ambush is a long-established military tactic, in which the aggressors take advantage of concealment and the element of surprise to attack an unsuspecting enemy from concealed positions, such as among dense underbrush or behind hilltops...

es, sabotage

Sabotage

Sabotage is a deliberate action aimed at weakening another entity through subversion, obstruction, disruption, or destruction. In a workplace setting, sabotage is the conscious withdrawal of efficiency generally directed at causing some change in workplace conditions. One who engages in sabotage is...

, raid

Raid (military)

Raid, also known as depredation, is a military tactic or operational warfare mission which has a specific purpose and is not normally intended to capture and hold terrain, but instead finish with the raiding force quickly retreating to a previous defended position prior to the enemy forces being...

s, the element of surprise, and extraordinary mobility

Mobility (military)

Mobility in military terms refers to the ability of a weapon system, combat unit or armed force to move toward a military objective. Combat forces with a higher mobility are able to move more quickly, and/or across more hostile terrain, than forces with lower mobility.Mobility is regarded as a...

 to harass a larger and less-mobile traditional army

Army

An army An army An army (from Latin arma "arms, weapons" via Old French armée, "armed" (feminine), in the broadest sense, is the land-based military of a nation or state. It may also include other branches of the military such as the air force via means of aviation corps...

, or strike a vulnerable target, and withdraw almost immediately.

The term means "little war" in Spanish

Spanish language

Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...

, and the word, guerrilla, has been used to describe the concept since the 18th century, and perhaps earlier.

The term guerrilla was used within the English language as early as 1809.