Severe turbulence
As 763 jock indicates "Severe" turbulence causes a very violent environment. In addition to what 763 jock has posted the capacity to accurately read instrumentation is likely to be impaired, due to the eyes being bounced around within the eye sockets. Often "severe" turbulence is reported when in fact it is "moderate" at worst. Under FAA definitions "Severe" turbulence is only less in intensity to "Extreme" which will likely cause structural damage to an aircraft.