Terrorism is, in its broadest sense, the unlawful use of intentional violence to achieve political aims, especially against civilians.[1] It is used in this regard primarily to refer to violence during peacetime or in the context of war against non-combatants (mostly civilians and neutral military personnel).[2] The terms "terrorist" and "terrorism" originated during the French Revolution of the late 18th century[3] but became widely used internationally and gained worldwide attention in the 1970s during the conflicts of Northern Ireland, the Basque Country, and the Israeli–Palestinian conflict. The increased use of suicide attacks from the 1980s onwards was typified by the September 11 attacks in New York City, Arlington and Pennsylvania in 2001.

United Airlines Flight 175 hits the South Tower of the World Trade Center during the September 11 attacks of 2001 in New York City

There are various different definitions of terrorism, with no universal agreement about it.[4][5] 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.[5][6][7][8][9] Varied political organizations have been accused of using terrorism to achieve their objectives. These include right-wing and left-wing political organizations, nationalist groups, religious groups, revolutionaries and ruling governments.[10] Legislation declaring terrorism a crime has been adopted in many states.[11] When terrorism is perpetrated by nation states, it is not considered terrorism by the state conducting it, making legality a largely grey-area issue.[12] There is no consensus as to whether terrorism should be regarded as a war crime.[11][13]

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.[14]