Source : Webster's Revised Unabridged Dictionary (1913)

Nightmare \Night"mare`\, n. [Night + mare incubus. See {Mare}
   incubus.]
   1. A fiend or incubus formerly supposed to cause trouble in
      sleep.

   2. A condition in sleep usually caused by improper eating or
      by digestive or nervous troubles, and characterized by a
      sense of extreme uneasiness or discomfort (as of weight on
      the chest or stomach, impossibility of motion or speech,
      etc.), or by frightful or oppressive dreams, from which
      one wakes after extreme anxiety, in a troubled state of
      mind; incubus. --Dunglison.

   3. Hence, any overwhelming, oppressive, or stupefying
      influence.

Source : WordNet®

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