Magic can describe any otherworldly force or energy. It can be manipulated by humans but only with tremendous danger to the user. For some greater magics it even requires the sacrifice of the soul. It is known to be practized in nearly every part of the human inhabited world. With advanced mathmatics and theoretical physics some grasp can be gained over the these forces.
There are numerous books dedicated to the study and practice of magic, the most infamous likely being the Necronomicon. Objects and even people may be infused with mystical energy. The extra-dimensional being Azathoth is known to supply his followers with magical energy. Most unearthly entities can only exist on our plane through the extensive use of magic. If not constantly charged with this magic they will be destroyed.
Professor Armitage described it as- "...It was mostly a kind of force that doesn't belong in our part of space; a kind of force that acts and grows and shapes itself by other laws than those of our sort of nature. We have no business calling in such things from outside, and only very wicked people and very wicked cults ever try to."