The Collaborative International Dictionary of English v.0.48:

Specter \Spec"ter\, Spectre \Spec"tre\, n. [F. spectre, fr. L.
   spectrum an appearance, image, specter, fr. specere to look.
   See Spy, and cf. Spectrum.]
   1. Something preternaturally visible; an apparition; a ghost;
      a phantom.
      [1913 Webster]

            The ghosts of traitors from the bridge descend,
            With bold fanatic specters to rejoice. --Dryden.
      [1913 Webster]

   2. (Zool.)
      (a) The tarsius.
      (b) A stick insect.
          [1913 Webster]

   Specter bat (Zool.), any phyllostome bat.

   Specter candle (Zool.), a belemnite.

   Specter shrimp (Zool.), a skeleton shrimp. See under
      Skeleton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

specter
    n 1: 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]
    2: a ghostly appearing figure; "we were unprepared for the
       apparition that confronted us" [syn: apparition, phantom,
       phantasm, phantasma, fantasm, specter, spectre]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "specter":
   Dracula, Frankenstein, Wolf-man, apparition, appearance, atua,
   bogey, bogeyman, bugaboo, bugbear, daemon, demon, eidolon,
   evil spirits, fantasy, fee-faw-fum, figure, form, frightener,
   genius, ghost, ghoul, hobgoblin, holy terror, horror, idolum,
   image, incubus, intelligence, monster, nightmare, ogre, ogress,
   phantasm, phantasma, phantasmagoria, phantom, presence, revenant,
   scarebabe, scarecrow, scarer, shade, shadow, shape, spirit,
   succubus, supernatural being, terror, umbra, vampire, vision,
   waking dream, werewolf, wildest dream, wraith