A plunge pool (or plunge basin or waterfall lake) is a deep depression in a stream bed at the base of a waterfall or shut-in. It is created by the erosional forces of cascading water on the rocks at formation's base where the water impacts.

The pool that forms at the base of a waterfall is geologically known as a plunge pool. These features are created by the powerful erosive forces of falling water and the rocks or debris (load) it carries. As the water crashes down, it performs "hydraulic action," scouring out the riverbed and creating a deep, often circular basin. Over time, the swirling motion of the water—sometimes called "eddying"—can cause the plunge pool to expand both in depth and width, often undercutting the cliff face itself and causing the waterfall to gradually recede upstream. In more casual or poetic contexts, these are sometimes referred to as "catchment basins" or "swimming holes," but "plunge pool" remains the standard technical term. In luxury travel and architecture, the term has been adopted to describe small, deep cold-water pools located at the foot of artificial water features or on private resort balconies, mimicking the refreshing, deep-water experience of their natural counterparts found in tropical rainforests or mountain ranges.