Having thus shown how State-centric and cosmopolitan concerns have influenced the process of criminalising (or creating a framework for criminalisation of) terrorism, this chapter will focus on the definitional process and principally, on how concerns that relate either to State interests or cosmopolitan purposes should shape the formulation of an international definition for terrorism. The analysis will start with the International Convention for the Suppression of the Financing of Terrorism, as it contains the first example of a treaty law definition that can be tracked down at an international level after the establishment of the UN. Secondly, the UN Draft Comprehensive Convention on Terrorism and the Appeals Interlocutory Decision of the Special Tribunal of Lebanon, as well as some regional instruments and domestic anti-terrorist law, will be compared and contrasted, with the purpose of tracing which definitional elements are the least debated and which ones remain contentious. The comparative analysis of these instruments will reveal some common ground that already exists (and can be used as a safe basis to build upon an international definition) as well as the two major contested elements of defining terrorism: (i) the inclusion of a political/ideological motive requirement and (ii) the exemption of activities carried out by particular groups or individuals from the definition. Subsequently, both these groups of elements will be put under the light of the ‘State sovereignty versus cosmopolitanism’ debate, highlighting their aspects that pertain to this debate and demonstrating how these aspects should be addressed in the context of finally formulating an international definition for terrorism.