A seance is an organized attempt to communicate with the dead.
Since it is impossible for the dead to communicate with the living, seances fall into two general categories:
1) Fraudulent charades given by fake mediums to fleece money from people. The "spectators" in these situations believe the manifestations to be real.
2) Theatrical performances given by magicians (generally those interested in Bizzare Magick). The spectators in these situations understand that they're watching a show.
There are several excellent texts for those interested in the latter category, among them Spirit Theater by Eugene Burger (1985) and Seance (19XX) by Scott Davis. Both are published by Kaufman and Company and available here: