Clairvoyant

Clairvoyance is the ability to get information about an object, location or event through the transfer of information from an external physical source. A form of extra sensory perception (ESP), it is often referred to as a sixth sense and is the art of seeing beyond the five senses that humans normally have. In its original meaning it directly translates as “clear seeing”.

A clairvoyant will detect energy that is not normally perceptible to humans such as radio waves. Most clairvoyants will have some kind of visual experience but it is not unknown to be able to hear or even taste something out of the ordinary.

Medium

The word medium usually applies to someone who can communicate with the dead using either ESP to relay messages from the spirits (channelling), by going into a trance or by allowing the spirit to use their body to communicate.

A medium will mentally see, hear or feel messages from the spirit, which are then relayed to another person.
If a reading is being done for a particular person then that person is known as the sitter.