Clairvoyance, Clairs or clear vision, is the ability to see things that you would not normally be able to see with your eyes, mainly, things, which are not physical (such as ghosts or spirits). The same with clairaudience, clairsentience and so on, this is where the Clair, in the name, comes from.
As for what they are and how they affect a medium’s ability we must first strive to understand what it is a medium is and what they are attempting to do in the first place.
A medium is an individual that is purported to have some kind of direct contact with the spirits of the dead. They act as the, “medium,” between the spirits of the dead and the family and friends of this spirit, hence the title of the practice, Mediumship.
Now the five Clairs are composed of sight, sound, touch, scent and emotion or inner knowing.
The first of these we have already mentioned, Clairvoyance. Clairvoyance is the practice whereby an individual can see things that often haven’t happened yet or things that happened a very, very long time ago (such as back in the civil war).
The second is Clairaudience, or clear hearing. This is the skill that allows a medium to hear voices or music or any other sounds from a different dimension or real (such as the voices of the dead from the spirit world).
The third Clair sense is Clairsentience, or clear feeling. This medium ability is the ability to intuitively feel many different kinds of information with the entire body. It is often described as one of the most subtle and difficult to master of the psychic techniques. It is used often to put the medium in a particular spirits shoes and see things uniquely from their perspective.
The fourth Clair sense is Clair tangency. Clear touch, as it is also known, is the ability to gain knowledge, sometimes of the future but most typically of the past from touching an object of some historical significance, sometimes the item is ordinary and unremarkable. This ability is also known as Psychometry.
The last Clair sense skill is called Clair scent, as you might suspect, this means clear smelling or clear scent. It is the ability of a medium to smell things coming in from different worlds or dimensions (such as the world of the dead). It is considered the hardest of the Clair skills to master because scents are everywhere and the medium must sort out which are from the world they know and which are from the spirit world. In addition humans do not have particularly sensitive noses in comparison to most other animals.