The word “terror” entered Middle English in the early 15th century, from the Old French terreur, from the Latin terrere (“to frighten”). The word “terrorism”, however, entered English in a specific context, as a translation of the word in French – composed of the classical Latin terror and the French -isme suffix – during the period known as the Reign of Terror (La Terreur) in France (1793-94).
At that time, the country was ruled by a Jacobin faction whose leaders declared terror to be “the order of the day” – alluding to terror cimbricus, the state of panic in ancient Rome ahead of the anticipated arrival of Cimbri tribal warriors in 105BC – and made a policy of punishing, usually by execution, anyone deemed to oppose the revolution.