Paul Pillar, a former deputy chief of the CIA’s Counter-terrorist Centre, argues that there are four key elements to terrorism: -
1. It is premeditated: planned in advance rather than an impulsive act of rage.
2. It is political: not criminal, like the violence that groups such as Mafia use to get money, but designed to change the existing political order.
3. It is aimed at civilians: not at military targets or combat ready groups.
4. Sub-national groups carry it out: not the army of a country.
See Pillar's Terrorism and U.S. Foreign Policy (pdf) for further reading.
In interpreting terrorism, the FBI uses this: "Terrorism is the unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives."
The U.S. Department of State defines "terrorism" to be "premeditated politically-motivated violence perpetrated against non-combatant targets by sub-national groups or clandestine agents, usually intended to influence an audience.
Mapped! Germany is one of more than 45 countries around the world where the threat from terrorism is rated "high" by the Foreign Office.
Find out other destinations with a "high" threat of terror on The Telegraph.