The following is an extract from the 2008 book Wicca Magickal Beginnings which Sorita d’Este co-authored with David Rankine.:
Aleister Crowley is often credited with inventing the use of the term magick (with a k) as opposed to magic. The addition of the k increases the magickal potency of the word as k is the eleventh letter of the alphabet and eleven is a number considered to contain a lot of inherent magical power within the western magickal system (being the one beyond ten). Crowley himself also emphasised that he used the spelling of magick in order to differentiate the term from the variety of magic practised by stage performers. It is however also interesting to note that the use of both ‘magick’ and ‘magic’ occurs in medieval and renaissance grimoires, as well as other documents of the time, as there was no standardisation of spelling. For example, in the book History of Magick published in 1657 we find:
“From these four different wayes, we infer four kinds of Magick: Divine, relating to the first, Theurgick, to the second, Goetick, to the third, and Naturall, to the last…”
Richard Boulton’s two-volume work of 1715-16, A Compleat History of Magick, Sorcery and Witchcraft continued this use with a ‘k’. Likewise in a completely non-esoteric context, we find it used in a scientific document published in 1811: “To describe the construction of the Magick Lantern”
Throughout the nineteenth century, both spellings continued to be used frequently, and as Crowley was a well-read man he would certainly have been familiar with the alternative spelling. So one could say that Crowley did popularise the Magick spelling, but also that he revived the use of this manner of spelling for the word (which was still after all widely in use a few decades earlier), whichever way – he certainly did not invent it. On a personal basis, we use this spelling to differentiate and emphasise, like Crowley, the difference between stage conjuring and the work done within the Western Mystery Traditions, which includes that of Wicca.
 Institutes of Natural Philosophy, William Enfield, Alexander Ewing, Samuel Webber, 1811