A clairvoyant is someone who claims to have supernatural senses that others do not, usually either a.) the ability to see the future or b.) the ability to talk to the dead.
Example for a.) The clairvoyant predicted that there would be a big earthquake tomorrow.
Example for b.) The clairvoyant was able to reveal secrets from beyond the grave.
The word itself can be a noun (referring to the person), or an adjective (i.e. "a clairvoyant prediction"). The noun to describe the act itself is "clairvoyance."