Just an opinion, but most of us have probably never seen anything more than "moderate turbulence" as far as the definition is concerned.

Definitions of turbulence levels per http://www.pnl.gov/atmospheric/docs/...appendix_c.pdf

Chop - Refers to a light or moderate climatic condition of washboard-type fluctuations occurring at rapid
and frequent intervals.

Light turbulence - An airborne condition where the aircraft is subjected to irregular perturbations of small
magnitude from smooth slight and occupants may be required to use seat belts, but objects in the aircraft
remain at rest.

Moderate Turbulence - An airborne condition where the aircraft is subjected to irregular perturbations of
uncomfortable magnitude from smooth flight and the occupants are required to use seat belts and are
occasionally thrown against the belt; unsecured objects in the aircraft may move about. This condition
requires the constant attention of an experienced pilot for safe flight.

Severe Turbulence - An airborne condition where the aircraft is subjected to irregular perturbations of
high magnitude from smooth flight and may be at the limits of control boundaries. Occupants can be
expected to be thrown violently against the seat belt and objects not secured are tossed about.

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RFTraveler