Medium

A medium is a mortal who possesses the ability to sense, communicate with, or channel the ghosts of the dead. Mediums often possess other psychic powers or some form of sorcery.

Old World of Darkness
Mediums in the Old World of Darkness are usually mortal characters (or rarely supernaturals) who possess the Medium Merit; other related Merits have been published in various sourcebooks. Mediums are described more fully in the context of Wraith: The Oblivion in The Quick and the Dead and Mediums: Speakers with the Dead. The various projectors from Orpheus are not quite mediums, having attained a greater connection to the Underworld.

New World of Darkness
Mediums in the New World of Darkness are described in the "Psychic Phenomena" chapter of World of Darkness: Second Sight. Here, "Mediumist" is a category of Merits available to mediums that describe the character's ability to make contact with disincarnate entities (usually ghosts, but rarely other spirits).