This is a general term that is used to describe a shifting series of imaginary or fantastic images as seen in a dream or fevered imagination. The term appears to have derived from a magic lantern experiment in 1802 presented by a Frenchman M. Philipstal. Variants of the term have been used to describe apparitions of phantoms, such as by Jean Baptiste Eyries in Fantasgoriana or Collection of the Histories of Apparitions, Spectres, Ghosts, etc. (1812).
This is the work from which Lord Byron read aloud to Percy Shelley, Mary Wollstonecraft (later Mary Shelley), Claire Clairmont, and J. W. Polidori on the night of June 16, 1816. Springing from Lord Byron’s reading was the suggestion that each member of the party should write a ghost story, which culminated in Mary Shelley writing Frankenstein that was first published in 1818. A.G.H.
Source: 9, 1278.