The Spanish word "guerrilla" is the diminutive form of "guerra" ("war"). The term became popular during the early-19th century Peninsular War, when the Spanish people rose against the Napoleonic troops and fought against a highly superior army using the guerrilla strategy. In correct Spanish usage, a person who is a member of a "guerrilla" unit is a "guerrillero" () if male, or a "guerrillera" () if female.
The term "guerrilla" was used in English as early as 1809 to refer to the fighters (e.g., "The town was taken by the guerrillas"), and also (as in Spanish) to denote a group or band of such fighters. However, in most languages guerrilla still denotes the specific style of warfare. The use of the diminutive evokes the differences in number, scale, and scope between the guerrilla army and the formal, professional army of the state.
Guerrilla warfare is a type of asymmetric warfare: competition between opponents of unequal strength. It is also a type of irregular warfare: that is, it aims not simply to defeat an enemy, but to win popular support and political influence, to the enemy's cost. Accordingly, guerrilla strategy aims to magnify the impact of a small, mobile force on a larger, more-cumbersome one. If successful, guerrillas weaken their enemy by attrition, eventually forcing them to withdraw.
Tactically, guerrillas usually avoid confrontation with large units of enemy troops, but seek and attack small groups of enemy personnel and resources to gradually deplete the opposing force while minimizing their own losses. The guerrilla prizes mobility, secrecy, and surprise, organizing in small units and taking advantage of terrain that is difficult for larger units to use. For example, Mao Zedong summarized basic guerrilla tactics at the beginning of the Chinese "Second Revolutionary Civil War" as: