Webster's definition of terrorism.

Terrorism. 1795: The systematic use of terror especially as a means of coercion.

This, I believe, is an adequate definition of terrorism in any context. This does not make religious zealotry terrorism. But terror may be a means of coercion, used by religous zealots. For instance the bombing of abortion clinics here in North America. No one in thier right mind with the least understanding of Christ's message would condone these actions. But they are done in his name. It is not religion which makes these people kill. It is something else.