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: Webster's Revised Unabridged Dictionary (1913)