A medium is one through whom the dead are held to communicate, and the office is defined by a claimed faculty and validated, in organised practice, by demonstration.
The claimed faculty
The medium claims the ability to receive and relay communications from the spirits of the dead, whether by speech, writing, or other means, acting as a channel between the living and the departed, so the office rests on a claimed faculty of contact rather than on any ordination or training in the ordinary sense, though practitioners often speak of developing the gift. The demonstration of the faculty, the delivering of messages held to come from the dead, is what establishes and sustains a medium standing among those who accept the practice.
Spiritualism and its scrutiny
The medium is central to Spiritualism and Spiritism, the movements that arose in the nineteenth century around the belief that the dead can be contacted and that they communicate through mediums, and in these organised forms the office is embedded in churches and societies with their own recognition of mediums. The claims of mediums have been the subject of intense scrutiny and repeated exposure of fraud, and they remain rejected by mainstream science and by many religious traditions, so an honest account records both the sincerity of many practitioners and believers and the long record of investigation and debunking. The figure has parallels in the possessed and the oracular of many cultures, but the medium in the strict sense belongs to the modern movements built around communication with the dead.