A Dictionary for Modern Times

Terrorism: ter·ror·ism  (těr’ə-rĭz’əm)

noun. “The unlawful use or threatened use of force or violence by a person or an organized group against people or property with the intention of intimidating or coercing societies or governments, often for ideological or political reasons.” American Heritage Dictionary, fourth edition, 2006.

According to Etymonline, the word itself dates from 1795, in specific sense of “government intimidation during the Reign of Terror in France” (1793-July 1794), from Fr. terrorisme (1798), from L. terror

“If the basis of a popular government in peacetime is virtue, its basis in a time of revolution is virtue and terror — virtue, without which terror would be barbaric; and terror, without which virtue would be impotent.” Robespierre in a speech in Fr. National Convention, 1794 [Robespierre, gentle reader, was, according to legend, the only person to be guillotined facing UP.]

Etym goes on to detail that: “General sense of “systematic use of terror as a policy” is first recorded in Eng. 1798. Terrorize “coerce or deter by terror” first recorded 1823. Terrorist in the modern sense dates to 1947, especially in reference to Jewish tactics against the British in Palestine — earlier it was used of extremist revolutionaries in Russia (1866); and Jacobins during the French Revolution (1795) — from Fr. terroriste. The tendency of one party’s terrorist to be another’s guerilla or freedom fighter was noted in ref. to the British action in Cyprus (1956) and the war in Rhodesia (1973). The word terrorist has been applied, at least retroactively, to the Maquis resistance in occupied France in World War II (e.g. in the “Spectator,” Oct. 20, 1979)”.

And since mid September 2001, to absolutely everyone in between.