severe turb- tubulence that causes large abrupt changes in altitude and/or attitude. It usually causes variations in indicated airspeed. Aircraft may be momentarily out of control.
After some years flying maritime patrol in a modified 60's airliner I have to laugh at how many pilots report light to moderate turbulence as severe. Just cause the pax don't like it doesn't make it severe.
Of course if your in an Airbus this does not apply!!!!!
And the worst I've had was extreme and it broke the jet but we were on a job about 40 west back in the cold war days so the rules were a bit different.