Vigilantism (sometimes "vigilianteism") is the act of enforcing an individual (or group's) code of justice or law enforcement outside of the law. This can often take the form of surveillance, attacking a suspected criminal, or making a citizen's arrest.
Several characters have been shown to engage in vigilantism in the Dick Tracy comic strip. The Crimestoppers Club is a type of vigilantism, though one of their guiding principles is not to endanger themselves (or contaminate evidence) by engaging criminals directly. This practice has been ignored on occasion by some of the Club's more daring members.
Moon Maid was shown to engage in vigilante acts, using her superhuman abilities to stop crimes that she encountered, such as robberies. This was often done with a disregard for the safety of her fellow citizens, but her acts were generally greeted with approval by Tracy and the police.
In later years, Tracy encountered the Cinnamon Knight, a costumed adventurer and sometimes-vigilante. Tracy discouraged these activities, urging the Cinnamon Knight (in his civilan identity) to join the police force if he was truly committed to enforcing the law.
The presence of vigilantes was sometimes used as a means of social commentary, as vigilantes could apparently accomplish things that the police (with their "confining regulations") could not.