Terror, n.
Etymology: [L. terror, akin to terrere to frighten, for tersere; akin to Gr. tras to tremble, to be afraid, Russ. triasti to shake: cf. F. terreur. Cf. Deter.]
1. Extreme fear; fear that agitates body and mind; violent dread; fright. Terror seized the rebel host. Milton.
2. That which excites dread; a cause of extreme fear. Those enormous terrors of the Nile. Prior. Rulers are not a terror to good works. Rom. xiii. 3. There is no terror, Cassius, in your threats. Shak.
Note: Terror is used in the formation of compounds which are generally self-explaining: as, terror-fraught, terror-giving, terror- smitten, terror-stricken, terror-struck, and the like. King of terrors, death. Job xviii. 14. -- Reign of Terror. (F. Hist.) See in Dictionary of Noted Names in Fiction.
Syn. -- Alarm; fright; consternation; dread; dismay. See Alarm.
Shades of meaning
Fernald has no entry of its own for terror; he lists it under alarm, fear.
Where he writes about it:
Apprehension, disquietude, dread, misgiving, and solicitude are in anticipation of danger; consternation, dismay, and terror are overwhelming fear, generally in the actual presence of that which is terrible, though these words also may have an anticipative force. — from alarm
A person of great timidity is constantly liable to needless alarm and even terror. — from alarm
Anxiety refers to some future event, always suggesting hopeful possibility, and thus differing from apprehension, fear, dread, foreboding, terror, all of which may be quite despairing. — from anxiety
Horror (etymologically a shivering or shuddering) denotes a shuddering fear accompanied with abhorrence or such a shock to the feelings and sensibilities as may exist without fear, as when one suddenly encounters some ghastly spectacle; we say of a desperate but fettered criminal, "I looked upon him with horror." Where horror includes fear, it is fear mingled with abhorrence. (See abhor.) Affright, fright, and terror are always sudden, and in actual presence of that which is terrible. — from fear
Fear may overwhelm, or may nerve one to desperate defense; fright and terror render one incapable of defense; fear may be controlled by force of will; fright and terror overwhelm the will; terror paralyzes, fright may cause one to fly, to scream, or to swoon. — from fear
Fright is largely a matter of the nerves; fear of the intellect and the imagination; terror of all the faculties, bodily and mental. — from fear
Dismay is more reflective, enduring, and despairing than fright; a horse is subject to fright or terror, but not to dismay. — from fear