An unconformity is a surface within a sequence of layers of sedimentary rock where rock deposition ceased, and erosion removed some previously formed layers before deposition resumed. Unconformities are significant in stratigraphy for understanding geological time gaps, the history of sedimentary basins, and tectonic events.
Reference: Miall, A. D. (1997). “The Geology of Stratigraphic Sequences.” Springer.