Derived from the French clair, meaning "clear", and voyance, meaning "vision", in the real world, it is refered to as the ability to gain information about an object, person, location or physical event through means other than the known human senses. The term is used to refer to the transfer of information that is both contemporary to, and hidden from, the clairvoyant. Unlike telepathy, that gains information directly from an external source.
In the Toaru Majutsu no Index universe, it is shown to be a simple ability that uses the eyes to "see through" objects, like X-ray vision. It, along with Precognition, Psychometry, Telepathy, and Psychokinesis, is one of the 5 attributes that is are measured in System Scans.