What is defined as a very rapid downward movement of rock or earth that may travel through the air?

A very rapid downward movement of rock or earth that travels through the air is typically referred to as a "fall." In geological terms, a fall describes a scenario where material such as rocks or debris detaches from a slope and free-falls, often influenced by gravity. This type of movement is characterized by a sudden and swift descent, distinct from other types of mass wasting processes which may involve more gradual movements or sliding along a surface.

On the other hand, earthflow pertains to a slower, viscous movement of saturated soil or sediment down a slope, which involves a flow rather than a fall. A slide generally involves a more cohesion-based movement along a particular plane of failure, where the material maintains some contact with the ground. Runout refers to the distance that fallen material travels away from the point of failure, particularly with respect to the area covered after impact, rather than the movement characteristics of the debris itself. This context reinforces why a fall is the most accurate term for describing a rapid descent through the air.