Downhill movement of large masses of soil or rock under the influence of gravity?

The answer is erosion. This includes the chemical and solution break up of rocks, and their transport to another place.

Aerial erosion is the process of moving sand and dusts by wind. Fluvial erosion is the same process but carried out by water, and this can move much larger particles. And the coasts are subject to erosion by the waves and the salt.