Oooh, fractals!
Magick is the supernatural ability to alter reality itself at will, possessed by Mages and other supernatural creatures such as werecreatures, vampires, and ghouls. The extra K at the end is to differentiate it from stage magic, which relies on non-supernatural arts such as misdirection and illusion to simulate true Magick.
Other alien races, such as the K'Nes, Vulthra, Bugs, and Caal also use magick, although from a fundamentally different philiosophical point of view.