A psyker
is an individual of any intelligent species in the Warhammer 40,000
universe who possesses some degree of psychic ability or powers.
Psychic powers are sometimes referred to as "sorcery" when they are gained or enhanced through interaction and bargaining with the malevolent entities who exist within the Empyrean. Psykers are also sometimes called " witches" in the Imperium of Man and among the Aeldari, particularly when such powers are used in a way that they show no regard for the dangers they may spawn for others or are used in pursuit of the agenda of the Chaos Gods. Psychic powers are sometimes referred to by the ignorant in the Imperium as " magic", also spelled " magick."
All psykers draw their powers from the extradimensional realm of psychic energy known as the Immaterium or the "Warp" that underlies and penetrates four-dimensional realspace and is the source of all psychic energy in the universe.
As such, psykers, particularly Human psykers whose minds are far less disciplined and easily tempted than their Aeldari counterparts, are often in danger of possession by the Daemonic entities of the Warp, the onset of insanity and ultimate corruption by the Ruinous Powers of Chaos.
