For such an integral part of the D&D world, magic, especially Arcane magic and Spell-like abilities, is scarcely defined in the rulebooks. Since magic and elementals are so tightly tied by convention, the place to talk about magic is here, in the book about elementals. The rules so far tell us a lot about how magic works, but nothing about what it is, why it works, or anything else of the sort. Most campaign settings make something up, although some leave it untouched, and because of this we can't give a definitive answer to this question. However, there are a number of answers as to what magic is not.