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]