A seance is a kind of magical ritual that involves summoning ghosts or spirits of people who have died. You and your friends might have a seance in an attempt to speak to Amelia Earhart.
Someone who believes in spirits that linger even after people's physical bodies are gone may also be enthusiastic about seances. A typical seance involves a group of people sitting around a table, sometimes including a medium who claims an ability to communicate with spirits and relay messages to their loved ones. In French, séance means "a sitting," and it's often simply used to mean "a session," like une séance de cinéma, "a movie session."