Vigilantism is theoretically and empirically important for political science, because it is closely related to core concepts like power, order, state-building, and collective action. Yet although political scientists are increasingly interested in vigilantism, our discipline has never developed a sustained, cumulative research agenda on vigilantism. I argue this is largely because existing definitions of vigilantism are contradictory, tautological, and not easily operationalized. This article cuts through the conceptual muddle, equipping and motivating political scientists to study vigilantism. First, I define vigilantism as the extralegal prevention, investigation, or punishment of offenses. Next, I provide a crisp conceptualization of vigilantism, and I situate vigilantism in relation to other concepts in political science. I conclude by addressing methodological and ethical issues in the study of vigilantism.