Undercutting refers to the erosion of the base of a slope or cliff, typically by water, which can lead to the collapse of the overhanging rock or soil. This process is significant in geomorphology for understanding the development of landforms, the stability of slopes, and the mechanisms of mass wasting.
Reference: Schumm, S. A. (1973). “Geomorphic Thresholds and Complex Response of Drainage Systems.” Fluvial Geomorphology, 69-85.