This winter I was on a flight in Europe during a huge winter storm. That's why the pilot announced for the flight that we will have some medium turbulence during the flight. However, it wasn't really shaking more than normal.
On the other hand, on my last flight over the Atlantic the pilot announced only small turbulence, but I'm not sure if I would have remained in my seat without my seatbelt.
Obviously, the forecast could just been wrong, but nevertheless I'm interested if there is any official classification of turbulence. So what is medium turbulence? What is severe turbulence?