This chapter examines the contested and politically charged nature of terrorism as a legal and conceptual category. It interrogates how terrorism, extremism, radicalisation, and jihad are defined across international, regional, and domestic frameworks, with particular attention to Malaysia’s counter-terrorism laws. By situating Malaysian legal responses within broader global debates, the chapter exposes the consequences of definitional ambiguity for criminalisation, preventive powers, due process, and human rights. It argues that the instability of terrorism definitions is not merely theoretical but has tangible effects on law enforcement, judicial practice, and individual rights.