Most air passengers will never be (un)lucky enough to be in severe turbulence, as properly defined. Some "light" turbulence will be enough to spill drinks. Here's one set of definitions (Canadian):-

Light Turbulence
Aircraft Reaction
Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch. roll, yaw). Report as "Light Turbulence".
OR
Turbulence that causes slight, rapid and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Report as "Light Chop".

Reaction Inside Aircraft
Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking.

Moderate Turbulence
Aircraft Reaction
Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. it usually causes variations in indicated airspeed. Report as "Moderate Turbulence".
OR
Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as "Moderate Chop".

Reaction Inside Aircraft
Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.

Severe Turbulence
Aircraft Reaction
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. Report as "Severe Turbulence".

Reaction Inside Aircraft
Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking impossible.