This is how I see it:
Turbulence is defined in the Australian AIP as either light, moderate or severe. From the AIREP format, the following definitions are given:
Moderate Turbulence: There may be moderate changes in aircraft attitude and/or altitude, but the aircraft remainsunder positive control at all times; usually small variations in air speed; changes in accelerometer readings of 0.5g to 1.0g at the aircraft's centre of gravity; difficulty in walking; occupants feel a strain against seatbelts; and lose objects move about.
Severe Turbulence: Abrupt changes in aircraft attitudce and/or altitude; aircraft may be out of control for short periods; usually large variations in air speed; changes in accelerometer readings greater than 1.0g at the aircraft's centre of gravity; occupants are forced violently against seat belts; and lose objects are tossed about.
All very techo, so in the air I applied the following definitions:
Light Turbulence: I start to worry about my coffee spilling, I put the "fasten Seatbelt" sign on.
Moderate Turbulence: I gulp my coffee, and instruct the Flight Attendants to be seated, I begin to ask/look for clearance to smoother air.
Severe Turbulence: I slow the aircraft to the turbulence penetration speed, thank god everyone is strapped in, and demand clearance to smoother air.