A spiritualist is a person who believes that dead people can still send messages to living people, and who often tries to make that contact happen. The word covers two connected things: the belief itself (spiritualism) and the person who holds it. In practice, when people say spiritualist, they usually picture someone at a small meeting or in a private room, sitting with a family and passing on words they say come from someone who has died. That is very close to what people call a medium — the difference is that 'medium' names the job (the person the message passes through) while 'spiritualist' names the belief the person holds. Many mediums are spiritualists, but you could believe in spirit messages without ever doing it yourself. Spiritualism was hugely popular in Britain and America from about the 1850s to the 1920s, especially after wars, when many families had lost sons and wanted one more word from them; churches were built for it and some still hold services today. So the word is not only about a strange person in a dark room — it can be an ordinary church-going word. That said, in most modern writing the word carries a small cool distance: the writer is reporting what these people believe rather than sharing it. There is also a looser, older use of 'spiritualist' in philosophy, meaning someone who thinks reality is made of spirit rather than matter, but you will almost never meet that use. Note that spiritualist is also used as an adjective — 'a spiritualist church', 'a spiritualist meeting' — with no change of form.