A vigilante is a person who punishes criminals themselves, outside the law, usually because they believe the police cannot or will not do the job. That last part is the heart of the word: the vigilante is not a criminal for gain — they think they are doing justice — but they have no legal right to do it, and what they do is itself against the law. It is a countable noun: a vigilante, groups of vigilantes. It is also used before another noun to describe that kind of action: vigilante justice, a vigilante group, vigilante violence. How the word feels depends a lot on who is speaking. In news reports it is disapproving — it signals mob beatings, people killed on suspicion, no trial. In films and comics it is almost admiring: Batman is usually described as a vigilante, and so are many action heroes. Online there is now 'digital vigilante' or 'internet vigilante' for people who hunt down and expose others on social media. Because of the hero stories, students sometimes think the word simply means a brave helper — but in real-world writing it usually carries a warning inside it.