Magic is a type of supernatural power over natural forces.
So, what's the difference between natural, and supernatural forces, then?
I mean, in the normal D&D metaphysics, magical forces are a normal part of the multiverse. They just happen... well, naturally. So, the supernatural is itself, natural. A modern day physicist would say that there is no such thing as forces not of nature.
The very word - "supernatural" literally means "above nature", and in the Latin root, conveys the idea that the power comes from a deity or some other force humans are not born with.
So, then, we might say that in D&D - the supernatural is that which a D&D commoner (or a collection of commoners) cannot do.