What term is used to describe turbulence intensity that causes large, abrupt changes in attitude and indicated airspeed, making food service and walking impossible?

The term that is used to describe turbulence intensity causing large, abrupt changes in attitude and indicated airspeed, making food service and walking impossible, is "Severe." Severe turbulence can be characterized by drastic and erratic movements of the aircraft, which can lead to difficulties for crew and passengers. In such conditions, it becomes nearly impossible for cabin crew to move about the cabin safely or serve food, and passengers may need to be secured in their seats for their safety.

Other turbulence classifications such as "chop," "extreme," and "moderate" describe different intensity levels and effects. Chop generally refers to lighter and less disruptive turbulence compared to severe. Moderate turbulence does cause some changes in altitude and attitude, but not to the extent that services or walking become impossible. Meanwhile, extreme turbulence is even more intense than severe but lacks the common practical categorization used in real-time flight operations, as it could imply conditions that are rarely encountered. Hence, severe turbulence is the most fitting choice for the scenario described.