The English word militant is both an adjective and a noun, and it is generally used to mean vigorously active, combative and/or aggressive, especially in support of a cause, as in "militant reformers". It comes from the 15th century Latin "militare" meaning "to serve as a soldier". The related modern concept of the militia as a defensive organization against invaders grew out of the Anglo-Saxon fyrd. In times of crisis, the militiaman left his civilian duties and became a soldier until the emergency was over, when he returned to his civilian occupation.

The current meaning of militant does not usually refer to a registered soldier: it can be anyone who subscribes to the idea of using vigorous, sometimes extreme, activity to achieve an objective, usually political. A "militant [political] activist" would be expected to be more confrontational and aggressive than an activist not described as militant.

Militance may or may not include physical violence, armed combat, terrorism, and the like. The Trotskyist Militant group in the United Kingdom published a newspaper, was active in labour disputes, moved resolutions in political meetings, but was not based on violence. The purpose of the Christian Church Militant is to struggle against sin, the devil and ". . . the rulers of the darkness of this world, against spiritual wickedness in high places" (Ephesians 6:12), but it is not a violent movement.