For a 12" radius -
If you draw a perfect circle around a center point that is twelve inches from the edge of the circle, the curve created is equal to the curvature of the fretboard.
This applies to a 10" radius, a 7.5" radius, a 15" radius, etc. Obviously, the closer the edge of the circle is to the center, the smaller the circle and the more radical the curve, so a 7.5" radius (like found on vintage Fender boards) is quite round, while a 21" radius (like you might find on an Ibanez Wizard neck) is only very slightly curved, and therefore known as a "flat" neck.