‘Violent extremism’ is rarely defined: neither the United Nations nor the European Union has an official definition. USAID defines it as “advocating, engaging in, preparing, or otherwise supporting ideologically motivated or justified violence to further social, economic or political objectives”. However, this apparently simple and obvious statement conceals a great deal of controversy and uncertainty. Is violent extremism, by definition, something carried out by non-state actors? In conflict situations, how can we differentiate violent extremists from other, more legitimate conflict actors? Does violent extremism always have to be ideological – can it, for example, be criminal, or even purposeless? Is ‘violent extremism’ merely a synonym for ‘terrorism’? More fundamentally, are terms like ‘extremism’ relative – in which case does ‘violent extremism’ mean different things to different people? These are not merely academic questions: what we call a phenomenon helps determine how we see it and what we do in response to it.