Quote Originally Posted by Azelor View Post
It exist in the nature and flow naturally in the world.
^ I like this one, with the addition that it's not necessarily equally distributed. Some areas might be deprived of magical potential, while others are overflowing with it.

I also don't see any need to define the origins much further than that. It's fun to leave room for interpretation so that we might have different cultures/schools of magic develop in the world and argue over the details. Better to focus more on questions of what magic can do and how it can be accessed.