terrorism: the use of violence and threats to intimidate or coerce, especially for political purposes.
I would vote that it depends on the reason of the bombing. If the bomber had the belief that abortion is murder, that a crime was being committed and ignored by society in the clinic, and the bomb was intended to stop these acts, then no, that's not terrorism - in fact, closer to an act of a vigilante. The intent was not to intimidate or coerce; the intent was strictly to stop something from happening using force.
Nonetheless, the bomber is a nutter who deserves life in jail, despite my own anti-abortion views.