Emily Dickinson Lexicon

IN'CU-BUS, n. [L. from incubo, to lie on.]

1. The nightmar; suffocative anhelation, with a sense of external pressure upon the chest, often seeming to be that of some hideous monster, and with tremor or violent struggle, transitory; most commonly occurring during sleep, though sometimes during wakefulness.
2. A demon; an imaginary being or fairy. Bp. Hall.