Terrorism in the modern sense is violence or other harmful acts committed, or threatened, against civilians for political or other ideological goals.

Most definitions of terrorism include only those acts which are intended to create fear or "terror," are perpetrated for an ideological goal, and deliberately target or utterly disregard the safety of non-combatants. Many definitions also include only acts of unlawful violence.

Terrorism has been used by numerous political organisations to further their objectives: both right-wing and left-wing political parties; nationalistic and religious groups; revolutionaries and ruling governments. The presence of non-state actors in widespread armed conflict has created a controversy regarding the application of the laws of war.