Terrorism is the systematic use of terror especially as a means of coercion." There is no internationally agreed definition of terrorism. Most common definitions of terrorism include only those acts which are intended to create fear (terror), are perpetrated for an ideological goal (as opposed to a lone attack), and deliberately target or disregard the safety of non-combatants. Some definitions also include acts of unlawful violence and war. A common opinion about terrorist groups, especially after the 'Global War on Terror' began after 9/11/2001, is that the majority of terrorist attacks are due to Islamic-extremists or radical religious groups. The 2001 attack of the World Trade Center and the hijacking of four passenger jets are a very well known and a well documented example of Islamic terrorism in recent memory.

Terrorism is also a form of unconventional warfare and psychological warfare. The word is politically and emotionally charged,Hoffman, Bruce "Inside Terrorism" Columbia University Press 1998 ISBN 0-231-11468-0. Page 32. See review in The New York TimesInside Terrorism and this greatly compounds the difficulty of providing a precise definition. One 1988 study by the US Army found that over 100 definitions of the word "terrorism" have been used.Dr. Jeffrey Record, Bounding the Global War on Terrorism(PDF) A person who practices terrorism is a terrorist.

Terrorism has been used by a broad array of political organizations in furthering 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 controversy regarding the application of the laws of war.

An International Round Table on Constructing Peace, Deconstructing Terror (2004) hosted by Strategic Foresight Group recommended that a distinction should be made between terrorism and acts of terror. While acts of terrorism are criminal acts as per the United Nations Security Council Resolution 1373 and domestic jurisprudence of almost all countries in the world, terrorism refers to a phenomenon including the actual acts, the perpetrators of acts of terrorism themselves and their motives. There is disagreement on definitions of terrorism. However, there is an intellectual consensus globally, that acts of terrorism should not be accepted under any circumstances. This is reflected in all important conventions including the United Nations counter terrorism strategy, the decisions of the Madrid Conference on terrorism, the Strategic Foresight Group and ALDE Round Tables at the European Parliament.