The FBI uses this definition:"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". Motive is a huge factor. If it's a disgruntled employee, I would call it a mass murder. If it was part of a larger agenda, then I would call it an act of terror.