A formation is a fundamental unit of lithostratigraphy, representing a body of rock with a distinct set of characteristics that distinguish it from adjacent rock units. Formations are used to map and describe the geology of an area, and they are key in understanding the distribution of rock types, sedimentary environments, and geological history.
Reference: Salvador, A. (1994). International Stratigraphic Guide: A Guide to Stratigraphic Classification, Terminology, and Procedure. Geological Society of America.