According to standard aviation reporting terminology, how is 'severe turbulence' defined?

1. Turbulence that causes large, abrupt changes in altitude or attitude, and usually large variations in indicated airspeed, during which the aircraft may be momentarily out of control.
2. Turbulence that causes slight, rapid, and rhythmic bumps without appreciable changes in altitude.
3. Turbulence that requires the flight crew to immediately don oxygen masks and initiate an emergency descent.
4. Turbulence that occurs only in clear air above 20,000 feet and is accompanied by structural deformation.