\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.