Rebels, insurgents, paramilitaries, separatists, militants, guerrillas, insurrectionists, fundamentalists... are these all terrorists? Or does terrorism claim its own exclusive niche? The exasperating inability to define terrorism is betrayed in the UN 2006 Global Counter-Terrorism Strategy - "we, the States Members of the United Nations...strongly condemn terrorism in all its forms and manifestations, committed by whomever, wherever and for whatever purposes".

The UN has been striving for decades to find a wording for terrorism which, instead of "all its forms and manifestations", narrows down to a specific method of conducting violence which can be condemned regardless of the circumstances. The absence of an agreed definition matters for many reasons. It blocks the possibility of referring terrorist acts to an international court, as for genocide and other war crimes; it leaves individual countries free to outlaw activity which they choose to classify as terrorism, perhaps for their own political convenience; and crucially it enabled the Bush administration to conjure in the public mind parallels between the 9/11 destruction of the World Trade Center and the Iraqi regime of Saddam Hussein. The vocabulary of terrorism has become the successor to that of anarchy and communism as the catch-all label of opprobrium, exploited accordingly by media and politicians.