Terrorism is the systematic use of terror, often violent, especially as a means of coercion. In the international community, however, terrorism has no legally binding, criminal law definition. Common definitions of terrorism refer only to those violent acts which are intended to create fear (terror); are perpetrated for a religious, political or, ideological goal; and deliberately target or disregard the safety of non-combatants (civilians). Some definitions now include acts of unlawful violence and war. The use of similar tactics by criminal organizations for protection rackets or to enforce a code of silence is usually not labeled terrorism, though these same actions may be labeled terrorism when done by a politically motivated group. The writer Heinrich Böll and scholars Raj Desai and Harry Eckstein have suggested that attempts to protect against terrorism may lead to a kind of social oppression.

The word "terrorism" is politically and emotionally charged, and this greatly compounds the difficulty of providing a precise definition. Studies have found over 100 definitions of “terrorism”. The concept of terrorism may be controversial as it is often used by state authorities (and individuals with access to state support) to delegitimize political or other opponents, and potentially legitimize the state's own use of armed force against opponents (such use of force may be described as "terror" by opponents of the state).

Terrorism has been practiced by a broad array of political organizations to further their objectives. It has been practiced by both right-wing and left-wing political parties, nationalistic groups, religious groups, revolutionaries, and ruling governments. An abiding characteristic is the indiscriminate use of violence against noncombatants for the purpose of gaining publicity for a group, cause, or individual. The symbolism of terrorism can leverage human fear to help achieve these goals.