In trying to answer the nature of magic in Equestria, we must first, obviously, consider the question, ‘What is magic?’. By the definition in Hard Fact’s dictionary, magic is: an unknowable, pearmating force, used to affect the enforcement of the user’s will upon reality. This is a good starting point. However, as my own studies have shown, Friendship is also Magic, and as a pegasus explained to me in Fillydelphia, Music can also be Magic. But most ponies commonly associate magic with the unicorn and their natural, active ability to manipulate things as if with another limb.
This is the unicorn’s most basic ability, psychokinesis. Manifesting this is dependent upon a certain thaumic flux passing through the unicorn’s horn, leading to the shimmering glow surrounding it. The stronger the glow, the more flux is passing through. Thus, we can here after assume that there is a natural ‘magic’ or thaumic flux feild surrounding Equestria at all times. The famed scholar Starswirl the Bearded also proposed such an idea in the pre-classical period. We shall call this natural field as ‘Base’ magic, for it is the most basic kind of magic available to us.
Friendship as Magic is a rather abstract concept, even by magic standards. The magic of Friendship is felt by everypony, yet can only ever be harnessed to its true and full potential by the artifacts known as the ‘Elements of Harmony’. Since their rediscovery, countless scholars have theorised that in its pure and untempered form, Friendship as a source of magic may well over power any and all power sources, and speaking from personal experience, I can say that this is likely not far off the truth. Everytime myself and my friends have used the elements, we have been literally overloaded with magical power, to the point of near magical burnout, and yet, it never seems too harm us.
Music as Magic is an unusual case in the exotic means of creating magic, as it is only able to be harnessed by a select few, who undergo rigorous training to enable them to wield it as effectively as they do. Primarily, it manifests as solid concussive blasts, matching the pony’s natural aura. It is also the only one to have been witnessed to have a perfect 1-to-1 conversion ratio in either direction.
Thus, the answer to the question ‘what is magic?’ quickly begins to get complicated. However, I have narrowed it down to this: magic is anything that provides enough energy for the user to force reality to bend to their will temporarily. Of course, I have only considered normal, Equestrian magic here. There are other kinds, but they are far less widespread, though at times, infinitely more powerful.