This report summarizes, and builds on, some of the author’s previous conceptual work. It approaches the definition of terrorism from five angles: (i) by focusing on the history of terrorism;(ii) by focusing on the psychology of ‘terror’ (the threat and fear factor); (iii) by focusing on forms of political violence other than terrorist violence; (iv) by focusing on the terrorist act; and (v) by focusing on the terrorist. Subsequently it addresses the question who should have definition power? The author looks at how terrorists, victims of terrorism, religious authorities, mass and social media, national governments, the United Nations, and members from academia have tried to define terrorism. In his conclusion, the author pleads for a narrow definition of terrorism. The main body of the text is followed by a sample of definitions of terrorism and a bibliography of books, book chapters, and articles on the subject.