specter in WordNet English dictionary

1. a ghostly appearing figure
   "we were unprepared for the apparition that confronted us"
   apparition  phantom  phantasm  phantasma  fantasm  specter  spectre 
   
2. a mental representation of some haunting experience
   "he looked like he had seen a ghost"; "it aroused specters from his past"
   ghost  shade  spook  wraith  specter  spectre