Terror is a status effect in the online main series games. The status effect freezes targets in fear, preventing them from acting for a short time. It also freezes the target's character model until the effect wears off. There is no way to remove the terror status effect, only time will cure its effects.
The spell Jettatura, learned by Blue Mages from Hippogryphs, is the only player spell that inflicts this status. The Beastmaster Job Ability Feral Howl also gives this effect, and it is an additional effect from the Greatsword Nightfall. The effect can also be inflicted by the abilities of three allied NPCs: Duskraven's Warden of Terror, Leonyne's Spine Chiller, and Rongelouts's Tongue Lash. Terror is the additional effect of several enemies in Final Fantasy XI, including Baobhan Sith, Brigandish Blade, Biast, Dame Blanche, Manes, and Shii. Terror can be inflicted by the Enemy Abilities Absolute Terror, Daunting Hurl, Dissipation, Dreadstorm, Faze, Gagnrath, Infinite Terror, Jettatura, Sangetall, Nightmare Mandragora's Scream, Thundris Shriek, Void of Repentance, and Vortex.