The Collaborative International Dictionary of English v.0.48:

Nightmare \Night"mare`\ (n[imac]t"m[^a]r`), n. [Night + mare
   incubus. See Mare incubus.]
   1. A fiend or incubus formerly supposed to cause trouble in
      sleep. [archaic]
      [1913 Webster]

   2. A trerrifying or oppressive dream characterized by a sense
      of helplessness in the face of danger, extreme uneasiness
      or discomfort (as of weight on the chest or stomach,
      impossibility of motion or speech, etc.) or extreme
      anxiety, from which one wakes in a troubled state of mind.
      [1913 Webster +PJC]

   3. Hence: Any overwhelming, oppressive, or terrifying
      experience resembling a nightmare[2] especially in the
      inability to escape from an unpleasant situation.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

nightmare
    n 1: a situation resembling a terrifying dream [syn:
         nightmare, incubus]
    2: a terrifying or deeply upsetting dream

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "nightmare":
   Dracula, Frankenstein, Wolf-man, bad dream, bogey, bogeyman,
   brown study, bugaboo, bugbear, clawing, cruciation, crucifixion,
   daydream, dream, fantasy, fee-faw-fum, frightener, ghost, ghoul,
   hell, hell upon earth, hobgoblin, holocaust, holy terror, horror,
   incubus, laceration, lancination, martyrdom, monster, ogre, ogress,
   passion, persecution, phantom, pipe dream, purgatory, rack,
   revenant, reverie, scarebabe, scarecrow, scarer, specter, succubus,
   terror, torment, torture, vampire, vision, werewolf