An individual possessing mental facilities sufficient enough to directly affect the physical world by force of will alone.
Contrary to popular belief, true invasive telepathy, what some might consider to be the most basic of psychic abilities, is in fact one of the rarest. Discrete abilities vary widely, though, and can be enhanced with training, enabling the psychic to perform fantastic or otherwise impossible feats like controlling local gravity, flash-freezing objects, complete mental domination, etc. This makes them natural candidates for receiving the Emperor's Artifacts and becoming Shades.
A psychic's potential is indicated by his or her Raw Mental Power, a figure from which real world comparisons can made, regardless of the disparity between abilities.