Short answer
“Magick” is a spelling of “magic” that the occultist Aleister Crowley adopted and popularised in the early twentieth century. He used it to set his own practice apart from stage conjuring and from what he saw as vague or superstitious ideas about magic. Today the K spelling is used mainly in Thelema and in other occult circles that follow him. It is not a different word, and ordinary dictionaries treat it as a variant.
An older spelling
Crowley did not invent the form. “Magick” appeared in English in earlier centuries as one of several spellings, as did other words ending in -ick before spelling settled. Writers in the early modern period used it, and Crowley’s contribution was to revive it and give it a specific meaning rather than create it from nothing.
Crowley’s definition
In Magick in Theory and Practice, published in the late 1920s, Crowley defined magick as “the Science and Art of causing Change to occur in conformity with Will.” His definition was deliberately broad. It covered anything from ritual to everyday acts of intention, as long as the person was acting on their will. This links closely to the core ideas of Thelema, where the discovery and carrying out of one’s True Will is central. For how this compares with other explanations, see magic theory.
Reasons for the K
Accounts of why he added the K give several reasons, and they are not all equally well documented.
- Distinguishing it from stage magic. Crowley explicitly said he wanted a separate word for his subject, to avoid confusion with conjuring and entertainment. This reason is widely cited and fits his writings.
- Numerology. K is the eleventh letter of the alphabet, and the number eleven had a significance in his system, associated with magick. This is also commonly reported.
- Symbolism. Some writers connect the letter K to other symbolic associations in his work. Details vary by source, so treat specific claims here with caution.
Because his explanations were in part symbolic, it is sensible to say that he gave a mix of practical and symbolic reasons without claiming a single official one.
How others use the spelling
- Thelemites and Crowley-influenced groups generally use “magick” consistently.
- Other occultists vary. Some use the K to signal the occult sense, others prefer plain “magic” and use context to make clear what they mean.
- Writers outside the community usually write “magic” for both, and most editors regard “magick” as nonstandard unless it is specifically a Thelemic or occult term.
Which spelling should you use?
- Use magic in general writing, for stage performance and for most historical contexts, including the practices described in natural magic and grimoires.
- Use magick when you are quoting Crowley, writing about Thelema, or following the usage of a particular group.
If you want an overview of the wider field, the occult explained is a good place to continue.