I don't know who came up with the term but I know RLM defines it in Ultimate Skiing if you have the book.
It is the angle between the ground's reaction force and the ski surface.
If the angle is 90 degrees you are just on the border between "edge locked" carve and "turning assisted by the sidecut".
I don't remember if Ron defined it as <90 degrees when the ski is edge locked or >90 degrees. Depends on how you measure the angle. I use the term "aggressive" to denote the edge locked side of the spectrum to avoid any misunderstanding.