So where does your magic come from, anyway?

What is magic? I mean, what is it? 

I've been thinking about this question a lot lately, as newer books get deeper into the magic of the Resonant Saga, and I spend my few idle fruit-season moments reading classic fantasy (the Wheel of Time [1]). I mean yeah, magic is people throwing fireballs or disappearing or whatever, but what is it? Where does it come from in these worlds that otherwise have gravity and thermodynamics and our other laws of physics? 

The answers in my favorite novels growing up never f...what  it?Time ).what it?