terror (n.)
early 15c., "something that intimidates, an object of fear," from Old French terreur (14c.), from Latin terrorem (nominative terror) "great fear, dread, alarm, panic; object of fear, cause of alarm; terrible news," from terrere "fill with fear, frighten," from PIE root *tres- "to tremble" (see terrible).

Meaning "quality of causing dread" is attested from 1520s. Sense of "a person fancied as a source of terror" (often with deliberate exaggeration, as of a naughty child) is recorded from 1883. Terror bombing first recorded 1941, with reference to German air attack on Rotterdam. Terror-stricken is from 1831. The Reign of Terror in French history (March 1793-July 1794) was the period when the nation was ruled by a faction whose leaders made policy of killing by execution anyone deemed an impediment to their measures; so called in English from 1801. Old English words for "terror" included broga and egesa.