Emily Dickinson Lexicon

NIGHT'MAR, n. [night and Sax. mara, incubus, nightmar. Mara may be from the root of merran, to stop, to hinder, (see Moor;) or it may be the Rabbinic מריא, an evil spirit or demon. The common spelling is erroneous, and tends to mislead the reader.]

Incubus; a sensation in sleep resembling the pressure of a weight on the breast, generally seeming to be that of some hideous monster or phantom, interrupting the sleep with violent struggle and tremor.