Vigilantes are people who voluntarily carry out duties usually done by Pro Heroes without paying attention to the law.
Due to the hero regulations and Quirk restriction laws, people are not allowed to perform heroic duties out on their own, thus, vigilantism is illegal.
The origins of Heroes can be traced back to vigilantes. When Quirks started to manifest, there was no organization controlling the people who used their Quirks to help others and take down the bad guys. Since these individuals were operating outside of the control of the government and law enforcement, they were declared vigilantes.
When the Pro Hero system was first put into place in Rhode Island, U.S.A. under the “Rhode Island New State Statute”, one hundred and eighty-nine vigilantes were affected by this statute, with only seven being accepted as official Pro Heroes. Nowadays most of the vigilantes have vanished.The Hero Public Safety Commission originally secretly enlisted vigilantes to do their dirty work, like disposing of corrupt Pro Heroes, until they became too organized for the vigilantes to properly deal with, so the Commission abandoned that idea and instead focused on recruiting Pros to handle this job.
Most recently, Knuckleduster, Pop☆Step, and The Crawler had been regularly active in their activities to help people and dispense justice as the vigilantes of Naruhata.
Tenya Ida, Izuku Midoriya, and Shoto Todoroki technically acted as vigilantes during their battle with the Hero Killer: Stain, due to them not possessing their hero licenses when they fought him.