vigilante
noun [ C ] uk/ˌvɪdʒ.ɪˈlæn.ti/ us/ˌvɪdʒ.əˈlæn.t̬i/
（尤指认为警方无法维持治安而自发组织起来的）治安会会员，治安维持者
a person who tries in an unofficial way to prevent crime, or to catch and punish someone who has committed a crime, especially because they do not think that official organizations, such as the police, are controlling crime effectively. Vigilantes usually join together to form groups.