Terrorism, in its broadest sense, is the use of intentional violence and fear to achieve political or ideological aims. The term is used in this regard primarily to refer to intentional violence during peacetime or in the context of war against non-combatants (mostly civilians and neutral military personnel). The terms "terrorist" and "terrorism" originated during the French Revolution of the late 18th century but became widely used internationally and gained worldwide attention in the 1970s during the Troubles in Northern Ireland, the Basque conflict, and the Israeli–Palestinian conflict. The increased use of suicide attacks from the 1980s onwards was typified by the 2001 September 11 attacks in the United States.

There are various different definitions of terrorism, with no universal agreement about it. Terrorism is a charged term. It is often used with the connotation of something that is "morally wrong". Governments and non-state groups use the term to abuse or denounce opposing groups. Varied political organizations have been accused of using terrorism to achieve their objectives. These include left-wing and right-wing political organizations, nationalist groups, religious groups, revolutionaries and ruling governments. Legislation declaring terrorism a crime has been adopted in many states. State terrorism is that perpetrated by nation states, but is not considered such by the state conducting it, making legality a grey area. There is no consensus as to whether terrorism should be regarded as a war crime. Separating activism and terrorism can be difficult and has been described as a 'fine line'.

The Global Terrorism Database, maintained by the University of Maryland, College Park, has recorded more than 61,000 incidents of non-state terrorism, resulting in at least 140,000 deaths, between 2000 and 2014.