There is much ambiguity associated with terror and terrorism. The Latin word terror came from the hypothetical Indo-European root TER, giving such words as terrible, deterrent, and perhaps turmoil. From its metathetical form, TRE, came other fearful words: the Latin tremere, giving tremble, tremendous, tremor, intrepid, and delirium tremens (first defined in 1813 by Dr Thomas Sutton as a form of delirium, not necessarily alcoholic, that was worsened by bleeding and improved by opium). Among tremulous flora and fauna are tremandra plants, with their shaking anthers, the gelatinous tremella fungi, and treron pigeons. And a vowel shift gives us tromometer, an instrument that detects earthly tremors.
Terror as a weapon has been wielded since early times, sometimes by insurgents, more often by governments. The early tyrants of ancient Greece and Sicily were benevolent rulers, who encouraged the development of democracy; the word tyrant probably came from the name of a Phrygian god and had nothing to do with terror. But by the 5th century bc despotic tyrants emerged who ruled by fear, and in the Greek city states that arose at that time tyrannicide was encouraged, although later Aristotle warned, in his Politics, that tyranny and extreme democracy could be equally degrading.
However, the first terrorists to be so called emerged only in the late 18th century. When a group of Dominican monks founded a house in the rue St Jacques in Paris they became known as Jacobins. Then when the Breton Club, a left wing political debating society, founded in Versailles in 1789, moved to an old Jacobin convent in the rue St Honoré, it became known as the Jacobin Club. Its members instituted the Reign of Terror, and so became known as terroristes, and their activities terrorisme—terms that first appeared in 1794 in François Noël (“Gracchus”) Babeuf's newspaper le Tribune du peuple. And, although there were already other verbs for what they did (atterrer, terrorifier), a new verb was coined—terroriser. First the terroristes terrorised the aristocrats; then, having seized power themselves, they terrorised other “enemies of the people,” which meant anybody they found despicable, including each other. Robespierre, for example, had Hébert, Danton, and Desmoulins guillotined and then—along with Saint-Just, Couthon, and others—was dealt with similarly by Barras. Then, adding further ambiguity, the Jacobins' Red Terror, as it was known, was followed by a royalist counterversion, the White Terror.
Although the word terrorist was invented in specific reference to the Jacobins, by the middle of the 19th century it came to have a more general meaning: “a member of a clandestine or expatriate organisation aiming to coerce an established government by acts of violence against it or its subjects” (Oxford English Dictionary); although nowadays, as ever, governments can also terrorise. Modern subtypes include ecoterrorists, bioterrorists, and agroterrorists. Intellectual terrorists terrorise figuratively, and in South Africa the word has even been used jocularly to mean a tourist.
But whom you call a terrorist depends on your point of view. Consistent with the equivocal dictum that one man's terrorist is another man's freedom fighter, the original terroristes wore the bonnet rouge, the Phrygian cap of liberty, or pileus, a brimless felt cap that was worn in ancient times by manumitted slaves and as a supposed cure for insanity. Today Western governments regard al-Qaeda as terrorists, but to al-Qaeda the terrorists are America and Britain. Joseph Conrad summarised this ambiguity well in The Secret Agent: “The terrorist and the policeman both come from the same basket. Revolution, legality—counter-moves in the same game.”