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From 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 especially in the inability to escape from an unpleasant situation. [1913 Webster +PJC]