This Article examines the evolution of the definition of terrorism at international law and tests the widely held view that international law does not provide a definition of terrorism. It contends that by abstracting from the common elements and themes present in the United Nations General Assembly and Security Council resolutions concerning terrorism, and the multi-lateral anti-terrorism conventions, treaties, and protocols, one can discern a core international law definition of terrorism. The Article then compares this definition to those in the domestic legal systems of the United States, United Kingdom, India, and New Zealand to determine whether (1) international law was influential in the drafting of these definitions or in the anti-terrorism legislative process generally and (2) these definitions are consistent with the international law definition discerned from the existing sources of international law relating to terrorism. It concludes that until a customary international law rule prohibiting terrorism emerges or a comprehensive terrorism convention is concluded, states should draw on the international law definition of terrorism when drafting their domestic antiterrorism legislation for legal and policy reasons, including to enhance the protection of human rights. The Article also examines the history of the development of the international law anti-terrorism instruments and the development of a comprehensive terrorism convention and model domestic legislation, and serves as a study of the implementation or incorporation of international law treaty obligations into domestic law in the context of terrorism.