How is the steepness of turn bank properly defined? In aviation we often refer to the bank of a turn as shallow, medium  or steep. I remember reading somewhere that the definition is not universal as to how many degrees it is, but it has to do with the airplane and whether the bank tends to roll back to level, remain where it is or continue to steepen. Does anyone know if this is a correct definition or have any references to share?
I don't know if this is only definition out there, but according to the FAA's Airplane Flying Handbook:


  Shallow turns are those in which the bank (less than approximately
  20°) is so shallow that the inherent lateral stability of the airplane
  is acting to level the wings unless some aileron is applied to
  maintain the bank.
  
  Medium turns are those resulting from a degree of bank (approximately
  20° to 45°) at which the air- plane remains at a constant bank.
  
  Steep turns are those resulting from a degree of bank (45° or more) at
  which the “overbanking tendency” of an airplane overcomes stability,
  and the bank increases unless aileron is applied to prevent it.