In Wheel of Time - it's ... the ability to manipulate the threads that make up reality/time, as though it were part of a fabric (I can't figure out how to better describe it) to produce different spells. In Mistborn - it's related to metals and how consuming/using them modifies the world of the user/the user In Stormlight Archive (still being written - Don't do what I do, wait for the series to finish) it's consumption of an energy to modify physics (again slightly different ideas than mistborn, but not sure how to say it better without spending more time). but what Sanderson seems to do best, is balance them. In many books I've read with similar concepts - the ability becomes too powerful/overwhelming on it's own. The rules change to fit the scenario. He seems to have a series of rules in place - where, yes we don't know everything that can be done, or when it will be done. When he introduces something new - it doesn't jarringly change how things worked in the past to fit the story - it's just the way it was, and we simply didn't know it.