Conventional historical periods – ‘classical’, ‘medieval’, ‘early modern’ etc. – help us as historians orient ourselves with respect to each other and to communicate what we do to a wider public. However, traditional periodisation is also dangerous. Giving a span of time a label tends to constrain our narratives within a set of assumptions. If we try to use the past to inform our understanding of the present, we may bring those assumptions forward, to identify or contrast with contemporary events in a way that has little to do either with the past or with the present. Terrorism is a historically and culturally contingent concept; it is modern, and it is Western. Past attitudes towards violence and who was entitled to use it were likewise very different from those that prevail in the modern West. As we write a history of terrorism, we should forgo both the use of conventional periodisation and the use of the term ‘terrorism’, even in a lowest-common-denominator sense, as a transtemporal, objective object of enquiry. This does not mean that we should jettison the word ‘terrorism’ altogether. We should rather view the word itself as having a history that embraces an evolving and shifting set of ideas, and that fits into a much older story about humanity’s views of order and disorder and its uses of violence and fear.