vigilance committee

noun

1. an unauthorized committee of citizens organized for the maintenance of order and the summary punishment of crime in the absence of regular or efficient courts.
2. History/Historical. (in the South) an organization of citizens using extralegal means to control or intimidate blacks and abolitionists and, during the Civil War, to suppress Union loyalists.

noun

1. (in the US) a self-appointed body of citizens organized to maintain order, punish crime, etc, where an efficient system of courts does not exist