Source : Webster's Revised Unabridged Dictionary (1913)

Doppelganger \Dop"pel*g["a]ng`er\, n. [G.]
   A spiritual or ghostly double or counterpart; esp., an
   apparitional double of a living person; a cowalker.

Source : WordNet®

doppelganger
     n : a ghostly double of a living person that haunts its living
         counterpart