Emily Dickinson Lexicon

MARE, n. [Sax. myra; G. mahre.]

1. The female of the horse, or equine genus of quadrupeds.
2. [Sax. mara, D. merrie, the name of a spirit imagined by the nations of the North of Europe to torment persons in sleep.] In medicine, sighing, suffocative panting, intercepted utterance, with a sense of pressure across the chest, occurring during sleep; the incubus. [It is now used only in the compound, nightmare, which ought to be written nightmar.]