The Collaborative International Dictionary of English v.0.48:

Wraith \Wraith\, n. [Scot. wraith, warth; probably originally, a
   guardian angel, from Icel. v["o]r[eth]r a warden, guardian,
   akin to E. ward. See Ward a guard.]
   [1913 Webster]
   1. An apparition of a person in his exact likeness, seen
      before death, or a little after; hence, an apparition; a
      specter; a vision; an unreal image. [Scot.]
      [1913 Webster]

            She was uncertain if it were the gypsy or her
            wraith.                               --Sir W.
                                                  Scott.
      [1913 Webster]

            O, hollow wraith of dying fame.       --Tennyson.
      [1913 Webster]

   2. Sometimes, improperly, a spirit thought to preside over
      the waters; -- called also water wraith. --M. G. Lewis.
      [1913 Webster]

WordNet (r) 3.0 (2006):

wraith
    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]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "wraith":
   Doppelganger, Masan, apparition, appearance, astral, astral spirit,
   banshee, co-walker, control, departed spirit, disembodied spirit,
   double, doubleganger, duppy, dybbuk, eidolon, etheric double,
   fantasy, fetch, figure, form, ghost, grateful dead, guide, hant,
   haunt, idolum, image, immateriality, incorporeal,
   incorporeal being, incorporeity, larva, lemures, manes,
   materialization, oni, phantasm, phantasma, phantasmagoria, phantom,
   poltergeist, presence, revenant, shade, shadow, shape,
   shrouded spirit, specter, spectral ghost, spirit, spook, sprite,
   theophany, unsubstantiality, vision, waking dream,
   walking dead man, wandering soul, wildest dream, zombie