A sudden rapid movement of soil and or rock down a slope.

These are landslides in the Himalayas.

Quite often the pieces of slope that slide become weakened during heavy rain when the water in the material helps to give it buoyancy. This is why the majority of landslides happen during or soon after rainstorms, or during periods of snowmelt. The word landslide is commonly used not just for soils that slide, but also for those that flow and which are also called mudflows.