Geological formation is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region (the stratigraphic column). It is the fundamental unit of lithostratigraphy, the study of strata or rock layers.
Source: Geological Survey of India (GSI). https://www.gsi.gov.in/webcenter/portal/OCBIS