n. The nightmare.n. An imaginary being or demon, supposed to be the cause of nightmare; especially, such a being of the male sex who was supposed to consort with women in their sleep.n. Figuratively, a heavy or oppressive burden; especially, a heavy weight on the mind; anything that prevents the free use of the faculties.n. [capitalized] In entomology, a genus of parasitic hymenopters of the family Braconidæ: synonymous with Microgaster of Latreille.