What is "psyker"

In the fictional universe of Warhammer 40,000, psykers are individuals with some form of psychic ability. They draw their power from the Warp, a parallel universe of psychic energy, and can use this energy for a variety of effects, such as reading minds, foretelling the future, healing injuries, or incinerating foes. Because the Warp is inhabited by daemons, all psykers are at a constant risk of being possessed.