The power to change things in conformity with will or desire. It is a function of focused consciousness accompanied by a force of Love intending change by reaching down into the Universal Field where everything exists as potential until affected by the operation of magic. This means that magic is happening all the time, but as magicians we have the opportunity and responsibility as co-creators to direct change in accordance with personal desire or what is called "The Great Plan," meaning no more and no less than whatever the underlying purpose of creation is.
As "low (or practical) magic" it is the intentional ritual action—supported by various physical correspondences with particular herbs, astrological factors, symbols, etc. lending strength to the visualized accomplishment through psychic powers—to make things happen as a materialization of desire.
As "High Magick" it is the intentional ceremonial action—supported by particular philosophical correspondence—to bring about self-development, including increased psychic skills, to the realization of the Whole Person. Which is what the Great Plan is all about.
"Magic, it is said, is the process of producing visible, physical results determined by upon by the trained will-thought of the magician who has found the way to communicate with the appropriate angelic Intelligences and win their collaboration. Magic has therefore been describes as the power to address the Gods in their own tongues."
Pg. 67, The Kingdom of the Gods, by Geoffrey Hodson, Theosophical Pub. House, Madras, India, 1953
"Your witchcraft…is very much a thing of the mind…the dominance of the witch’s mind over her surroundings."
Pg. 140 High Magic’s Aid. By "Scire" (Gerald Gardner), Michael Houghton, London, 1949
"Do you define magic as mumbo jumbo?"
"I always have."
"I define it as the practical application of knowledge of the little-understood powers of the human mind."
Pg. 104, The Winged Bull, by Dion Fortune, London, Williams & Norgate, 1935.