From David K. Rankine’s article on magick:
Some people ask why magick is spelt with a ‘k’ on the end. This is to distinguish it from magic, associated in the popular mind with illusionism and prestidigitation, stage magic. Magick is not about illusion, it is about creating real change, and the ‘k’ signifies this. K is the eleventh letter of the alphabet, i.e. the one beyond ten. Symbolically this is very powerful, as we work in base ten, and eleven represents the unseen, or hidden energies – the subtle energies of magick, eleven is considered to be the number of magick.