Vigilantism is the behaviour — the practice, the whole phenomenon — of ordinary people punishing criminals themselves instead of leaving it to the police and the courts. Where 'vigilante' is the person, 'vigilantism' is the thing they do, treated as a general problem or pattern. So you say 'the rise of vigilantism', 'a country with a long history of vigilantism', 'this is vigilantism, not justice'. It is uncountable and quite formal — you meet it in newspaper editorials, court reports, and academic writing about law and society, not usually in conversation. It nearly always carries disapproval: when someone calls an act vigilantism, they are saying it may feel like justice but it breaks the rule that only the courts may punish. A modern use you will see often is 'online vigilantism' or 'digital vigilantism' — crowds on social media finding someone's name and workplace and punishing them by shame. Say it vij-uh-LAN-tiz-um.

私刑主義、自行執法（的風氣）