The Convention for the Prevention and Punishment of Terrorism (Geneva, 1937) defines "acts of terrorism" as "criminal acts directed against a State or intended to create a state of terror in the minds of particular persons, or a group of persons or the general public."
Some broadly accepted definitions:
Terrorism is the use or threatened use of violence, usually against human life;
Terrorism is a means to achieve political goals which, in the view of those resorting to it, could not be attained by ordinary (lawful) means;
Terrorism is total war: the end justifies all means;
Terrorist acts are often directed at outsiders who have no direct influence on or connection with what the terrorists seek to achieve;
Terrorists often attack their victims indiscriminately .
The Council of Europe gives us a simple-to-test definition: An armed group that deliberately targets civilians.