1. A name invented for an exhibition of optical illusions: For phantasmagorias to be successful, optical phenomenons were produced chiefly by means of the magic lantern and were first exhibited in London in 1802.
2. Sometimes it was erroneously applied to the mechanism used: A phantasmagoria can be the device which produces such optical illusions, like a stereopticon or a magic lantern.
3. A shifting series or succession of phantasms or imaginary figures: A phantasmagoria can be seen in a dream or fevered condition, as called up by the imagination, or as created by a literary description.
4. Etymology: name of a "magic lantern" exhibition brought to London in 1802 by Philipstal, the name an alteration of a French version of phantasmagorie, said to have been coined in 1801 by French dramatist Louis-Sébastien Mercier, from Greek phantasma, "image" + second element probably a French form of Greek agora, "assembly". This may have been chosen more for the dramatic sound than any literal Greek sense.

The inventor of the word apparently wanted a fancy and startling term, and may have fixed on -agoria without any reference to a Greek lexicon.