Magick, in the broadest sense, is any act designed to cause intentional change. The spelling with the terminal "k" was repopularized in the first half of the 20th century by Aleister Crowley when he introduced it as a core component of Thelema.IAO131. Thelema & Buddhism in Journal of Thelemic Studies, Vol. 1, No. 1, Autumn 2007, pp. 26

"The Anglo-Saxon k in Magick, like most of Crowley's conceits, is a means of indicating the kind of magic which he performed. K is the eleventh letter of several alphabets, and eleven is the principal number of magick, because it is the number attributed to the Qliphoth - the underworld of demonic and chaotic forces that have to be conquered before magick can be performed. K has other magical implications: it corresponds to the power or shakti aspect of creative energy, for k is the ancient Egyptian khu, the magical power. Specifically, it stands for kteis (vagina), the complement to the wand (or phallus) which is used by the Magician in certain aspects of the Great Work."

For Crowley, the alternate spelling was used to differentiate it from other practices, such as stage magic. Magick is not capable of producing "miracles" or violating the physical laws of the universe (e.g., it cannot cause a solar eclipse), although "it is theoretically possible to cause in any object any change of which that object is capable by nature".

Crowley preferred the spelling magick, defining it as "the science and art of causing change to occur in conformity with the will." By this, he included "mundane" acts of will as well as ritual magic. In Magick in Theory and Practice, Chapter XIV, Crowley says:

What is a Magical Operation? It may be defined as any event in nature which is brought to pass by Will. We must not exclude potato-growing or banking from our definition. Let us take a very simple example of a Magical Act: that of a man blowing his nose.

Crowley saw magick as the essential method for a person to reach true understanding of the self and to act according to one's True Will, which he saw as the reconciliation "between freewill and destiny." Crowley describes this process:

One must find out for oneself, and make sure beyond doubt, who one is, what one is, why one is...Being thus conscious of the proper course to pursue, the next thing is to understand the conditions necessary to following it out. After that, one must eliminate from oneself every element alien or hostile to success, and develop those parts of oneself which are specially needed to control the aforesaid conditions.

Since the time of Crowley's writing about magick, many different spiritual and occult traditions have adopted the spelling with the terminal -k, but have redefined what it means to some degree. For many modern occultists, it refers strictly to paranormal magic, which involves influencing events and physical phenomena by supernatural, mystical, or paranormal means.