A medium is someone who is able to put in a sort of dreamlike state. This condition makes the person very sensitive to external influences. Apparently this person is a channel to messages between the living and the dead.
A medium is the center person of the so-called seance where information is communicated between the living and the dead. It also seems, that medium is able to collect information across time.
Many types of media exist. Some traffic information and messages. Other are very spectacular, like performing levitations.