There is no consensus in the literature about the nature of terrorism. The authors’ main claim is that this is ultimately the result of the coexistence of two senses of the term, the action and the actor sense, which are not fully congruent. Rather than trying to advocate a specific conceptualization, the authors provide in this article a map of the different ways in which scholars talk about terrorism. They identify first the set of terrorist actions and the set of (...) terrorist actors. Terrorist tactics are a variety of the power to hurt, based on the lack of military power. Terrorist groups are underground ones with no territorial control. When the two criteria meet, the core of terrorism exists: coercive violence perpetrated by underground groups. The ambiguity that surrounds terrorism is caused by two other possibilities: actors with some measure of territorial control adopting coercive tactics and underground actors adopting military tactics. Although it is not possible to remove this ambiguity in empirical research, scholars can at least identify it and analyze it. The authors illustrate the two senses of terrorism and their interaction by using the most comprehensive dataset on terrorist incidents, the Global Terrorism Database. (shrink)