The legal definition of terrorism has very high importance. This is so both because it determines which actions count as acts of terrorism, and hence who is regarded as a terrorist, but also because the definition of terrorism triggers a range of controversial extensions of police and prosecutorial powers. This article provides a comprehensive analysis of the central dilemmas that must be confronted to develop an adequate definition of terrorism. These include the questions of whether acts count as acts of terrorism only if they have a particular political purpose, what acts are sufficiently harmful to count as acts of terrorism, whether acts of terrorism are distinguished by the identity of those targeted, whether acts of terrorism are distinguished by the methods that terrorists use, and the importance of the identity of the terrorists themselves. It examines a range of legal definitions across jurisdictions in the light of the framework developed. It is argued that there is no prospect for a fully adequate definition of terrorism: the definition will inevitably be over-inclusive. A number of reasons explain this, but the central one is that courts lack the expertise to distinguish adequately legitimate political resistance from terrorism. Tensions that this problem has created in the case law are explored, not only in the criminal law, but also in the civil law. We then briefly examine the extent to which the discretion of officials can ameliorate this problem, highlighting the weakness of this solution. Overall, we conclude, disappointingly, that the standards that we ought to find in other areas of criminal justice cannot be met in the area of terrorism.

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