The word magic is used today in a plethora of ways to describe an even greater plethora of things, for the purposes of my grimoire I have landed on this definition: "Magic is the, as of yet unknown, mechanisms of the universe by which certain things happen, and somehow allow humans to interact with certain phenomenae currently unexplainable by science", this is quite vague but that will always be part of the result of working with the occult. Still, my definition focuses mainly on two things: The unknown (perhaps unknowable), and the possibility for human interaction, be it observation, influence or even outright control.
Due to the very nature of the occult, it is difficult (or perhaps even impossible) to outright prove it's existence, however, I'd like to point out some evidence toward it actually existing in no particular order: