n. A fiend or incubus formerly supposed to cause trouble in sleep.n. 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.n. Any overwhelming, oppressive, or terrifying experience resembling a nightmare{2} especially in the inability to escape from an unpleasant situation.