A definition of terrorism

The objective of this definition is to draw lines around a relatively consistent phenomenon, which has similar characteristics no matter who the perpetrators are, and to distinguish it from phenomena that have similar traits but are fundamentally different in conception and goal. 

Employing a consistent definition is useful for distinguishing how and why we respond to certain kinds of violence and removing certain biases (for instance, race and religion) that can make us less effective at countering terrorism as well as aggravating broader social problems. 

The key components of this definition are:

• Violence is public: Terrorism is is usually theatrical and is done to influence an audience. The word "terrorism" itself comes from the effect it is perceived to have on the audience, but these days, it is used for more than just fear. For instance, ISIS uses terrorism to provoke and enrage.
• Targets noncombatants: This includes both civilians and undeployed military personnel. Terrorist tactics can be used to fight deployed military personnel, but this is generally defined as insurgency or guerrilla warfare.
• Violence is not attributed to government: This distinguishes between terrorism and state actions under its monopoly on violence, or war. This does not excuse state violence, it just reclassifies it. When a state uses violence against noncombatants, it can be an act of war, or a war crime. Against its own population, it can be genocide, oppression or tyranny. "Not attributed" matters here, as some terrorism is state-sponsored. It is possible to imagine an overtly state-directed terrorist campaign against an adversary (the question of ISIS's state status is obviously relevant), but this is relatively rare and should probably be thought of in a war context. 
• Has political or ideological motive: This distinguishes an attack like Fort Hood or Charleston from being considered simply a mass shooting incident. It also distinguishes hate crimes sparked by uncomplex (i.e., nonideological) racism (such as a deadly fight with a longtime neighbor, or with a stranger in a bar, with an element of bias related to race or sexual orientation) from crimes designed to have a wider political impact. 

Buy the new book ISIS: The State of Terror by Jessica Stern and J.M. Berger.