Is it farther or further?
late 14c., "great fear," from Old French terreur (14c.), from Latin terrorem (nominative terror) "great fear, dread," from terrere "fill with fear, frighten," from PIE root *tre- "shake" (see terrible). Meaning "quality of causing dread" is attested from 1520s; terror bombing first recorded 1941, with reference to German air attack on Rotterdam. Sense of "a person fancied as a source of terror" (often with deliberate exaggeration, as of a naughty child) is recorded from 1883. The Reign of Terror in French history (March 1793-July 1794) so called in English from 1801. Old English words for "terror" included broga and egesa.