For the sake of developing and evaluating public policy decisions aimed at combating terrorism, we need a precise public definition of terrorism that distinguishes terrorism from other forms of violence. Ordinary usage does not provide a basis for such a definition, and so it must be stipulative. I propose essentially pragmatic criteria for developing such a stipulative public definition. After noting that definitions previously proposed in the philosophical literature are inadequate based on these criteria, I propose an alternative, which I call the 'group-target' definition and which distinguishes terrorism from other forms of violence by the distinctive principle of discrimination used by terrorists to identify legitimate targets. I argue that this definition meets the criteria for a satisfactory public definition, and suggest that based on it there is good reason to suspect the adequacy of anti-terrorism policies that rely predominantly on forceful interdiction of terrorists.