Severe turbulence:
-Turbulence that causes large, abrupt changes in altitude and/or attitude.
-It usually causes large variations in indicated airspeed.
-Aircraft may be momentarily out of control.
-Occupants are forced violently against seat belts or shoulder straps.
-Unsecured objects are tossed about. Food Service and walking are impossible.
Been in it once or twice and that was certainly enough. Really wakes you up.