in the modern sense, 1944, especially in reference to Jewish tactics against the British in Palestine -- earlier it was used of extremist revolutionaries in Russia who attempted to demoralize the government by terror (1866); and Jacobins during the French Revolution (1795) -- from French terroriste; see terror + -ist (also see terrorism).
The term now usually refers to a member of a clandestine or expatriate organization aiming to coerce an established government by acts of violence against it or its subjects. [OED]
The tendency of one party's terrorist to be another's guerilla or freedom fighter was noted in reference 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 (as in in the "Spectator," Oct. 20, 1979).