The Collaborative International Dictionary of English v.0.48:

Spook \Spook\ (sp[=oo]k), n. [D. spook; akin to G. spuk, Sw.
   sp["o]ke, Dan. sp["o]gelse a specter, sp["o]ge to play,
   sport, joke, sp["o]g a play, joke.]
   1. A spirit; a ghost; an apparition; a hobgoblin. [Written
      also spuke.] --Ld. Lytton.
      [1913 Webster]

   2. (Zool.) The chimaera.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spook
    n 1: someone unpleasantly strange or eccentric [syn: creep,
         weirdo, weirdie, weirdy, spook]
    2: a mental representation of some haunting experience; "he
       looked like he had seen a ghost"; "it aroused specters from
       his past" [syn: ghost, shade, spook, wraith,
       specter, spectre]
    v 1: frighten or scare, and often provoke into a violent action;
         "The noise spooked the horse"

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "spook":
   Masan, affright, agent, agitate, alarm, apparition, appearance,
   astral, astral spirit, awe, banshee, bogey, bug, case, character,
   control, convulse, curdle the blood, departed spirit, discompose,
   disembodied spirit, disquiet, distract, disturb, duck, duppy,
   dybbuk, eidolon, embroil, form, fright, frighten, funk, ghost,
   grateful dead, guide, hant, haunt, horripilate, idolum,
   immateriality, incorporeal, incorporeal being, incorporeity, larva,
   lemures, make one tremble, manes, materialization, obsess, oddball,
   oddity, oni, original, perturb, phantasm, phantasma, phantom,
   poltergeist, possess, presence, psych, quiz, raise apprehensions,
   revenant, scare, shade, shadow, shake, shape, shrouded spirit,
   specter, spectral ghost, spirit, sprite, spy, stagger, startle,
   terrify, terrorize, theophany, throw, throw into confusion,
   trouble, undercover man, unman, unnerve, unsettle, unstring,
   unsubstantiality, upset, vision, walking dead man, wandering soul,
   wraith, zombie