Shadow is also referred to in Roger Zelazny's Amber Series.
A shadow in this sence is generally considered a parallel universe, created by the powers of Chaos and Order. Any initiate of the powers casts his(her) Shadows if (s)he stays in the same Shadow long enough. A Shadow of a person is a being in a parallel universe that resembles (less and less with distance) the appearance and actions of the initiate of the powers. Theoretically, any immaginable place exists somewhere in the Shadows. Laws of nature differ widely between the shadows, so that the higher technological achievments appear to be working in the more limited number of Shadows. In order to shift or travel between Shadows a person must be a power initiate or follow one. One can also utilize the Shadow ways, Trumps and number of other artifacts and manifestations found in the Amber Universe.