Mediums were humans who could communicate with deceased beings on the Other Side as a result from being resurrected from a non-supernatural death. They are not considered a supernatural species, unlike the Doppelgängers and Supernatural Hunters who do not reside in the Other Side when they die.
Since the collapse of the Other Side, mediums can no longer contact the spirits of the dead, as the spirits have now moved on to other afterlife dimensions that are currently inaccessible to those in the living world.