Defining Magic is tough - so we went to the experts.
Unfortunately, there is no single, correct answer to this question. You will find answers as varied as the belief systems which incorporate magical elements and the people who practice within each system.
In seeking a definition of the term, perhaps we should look no further than the most mundane of sources:
It is the art of producing a desired effect or result through the use of various techniques as incantations that presumably assure human control of supernatural agencies or the forces of nature - Webster English Dictionary
Or we can defer to old philosophers and alleged magicians for meaning:
Aleister Crowly defined magick as:
the Science and Art of causing Change to occur in conformity with Will.
In The Lemegeton (Lesser Book of Solomon, the King), we find:
Magick is the highest, most Absolute, most Divine Knowledge of natural Philosophy, advanced in its works and wonderful operations by a right understanding of the inward and occult (hidden) virtue of things; so that true agents being applied to proper patients, strange and admirable effects will hereby be produced. Whence magicians are profound and diligent searchers into Nature; they, because of their skill, know how to anticipate an effect, the which to the vulgar shall seem to be a miracle.
Or one of these might well do:
"It is a Divine Science" -Henry Cornelius Agrippa
"Magic is the traditional science of the secrets of nature which has been transmitted to us by our ancestors of the priesthood of the Magi" -Eliphas Levi.
"Magic is the application of human Will (originating in the Soul) to the fast evolution of the Living forces of nature" -Gerard Encausse.
"The art of White magic is the art of using so-called Spiritual or invisible forces, to obtain visible results" -Franz Hartmann
For a contemporary definition, the best place to look might be the group alt.magick on Usenet. Here we we will find the term "magick" defined and accepted by the general consensus of those participating in discussion during 2000. (Seeing as these people spend the majority of their days discussing magic(k) , it is only logical they should know what it is, right?)
Some see magic as a kind of energy which pervades the cosmos. Others see it as a psychic tool by which we may influence the material world using symbols and ritual. Many see it as a means of coming to unite with the divine, some consider it to be the exercise of will or Will, or the manipulation of reality.
In other words, only the individual can determine what magick is or isn't for themselves. Magic is exactly what you believe it to be, for it requires your belief to be anything at all. As one wise and unknown Sage put it: "Magick is Within You!"