My working definition of terrorism is pretty simple: Terrorism is the deliberate murder of civilians or destruction of property in order to achieve a political objective. I think this definition works because it covers everything from cross-burnings and lynchings to the Weathermen bombings to the attack on the World Trade Center.
Viewed in this light, the murder of George Tiller is undoubtedly terrorism. It was done with an audience, as Tiller was in church with his wife in the room at the time — in other words, with the exact kind of theatricality terrorists require to achieve their objectives. The point was not simply to murder a man who performs abortions, it was to dissuade any one else from providing similar services.