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.

• a very bad or scary dream
       I had a nightmare that I tried to run but not could not move or breathe.
  
• (by extension) any bad, miserable, difficult or terrifying experience
       Cleaning up after identity theft can be a nightmare of phone calls and letters.