self-appointed individual or group that takes on the responsibility for maintaining law and order in a community, when the normal channels have become ineffective. Vigilante groups have been a feature of life in the troubled areas of Northern Ireland, for example, for over 20 years.
One who takes or advocates the taking of law enforcement into one's own hands
A vigilante is someone who takes enforcement of law or moral code into their own hands. The term vigilante stems from the its Spanish equivalent, meaning private security agents. It has most probably been introduced recently in English language from South-Western United States, in the same way as folkloric terms like "desperados" in the past.