Psyker
A Psyker is a Psionic individual in the Warhammer 40,000 universe, drawing their powers from the Immaterium. Certain species have a greater number of Psykers with greater Psionic powers. The Tau and Necrons lack any psionic abilities at all.
Human Psykers
By far the most powerful Psyker in existance is the Emperor who is for all intent and purpose, a god.
A certain precentage of Humans in the Warhammer 40,000 universe are born with psionic abilities. Generally these powers are quite low level and would otherwise be unnoticed, although in a few individuals these psionic powers can be formidable, if not as powerful as those of the Eldar.
In the Imperium, Psykers are placed under the administration of the Adeptus Astra Telepathica. All Psykers which are identified in the Imperium are transported aboard craft known as Black Ships to Holy Terra to either be consumed by the Emperor or to be bonded to him and serve as highly monitered soldiers on the battlefield. Psykers are generally feared and mistrusted in the Imperium as they are often especially vunerable to the corrupting powers of Chaos.
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Eldar Psykers
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Chaos Pskyers
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Ork Pskyers
Psionic orks are known as Weirdboyz. While they have minor innate powers, they become particularly powerful in the presence of large numbers of other orks. Weirdboyz absorb psychic energy from nearby orks, and they can release this energy for a variety of supernatural effects. Few weirdboyz have substantial control over their abilities, though, so they have "minderz" to control their exposure to other orks and help them direct their manifested powers (often by literally aiming the weirdboy's body at the enemy). Weirdboyz that absorb too much psychic energy from other orks can suffer severe consequences, up to and including head explosions.