(1) A general term applied in the well-logging industry to the external environment of the drilled well bore without stratigraphic connotation.

(2) The basic or fundamental rock-stratigraphic unit in the local classification of rocks, consisting of a body of rock (usually a sedimentary stratum of strata, but also igneous and metamorphic rocks) generally characterized by some degree of internal lithologic homogeneity or distinctive lithologic features (such as chemical composition, structures, textures, or gross aspect of fossils).

Formations may be combined in groups or subdivided into members and [[bed|beds]]. A formation name should preferably con sist of a geographic name followed by a descriptive lithologic term (usually the dominant rock type) or by the word formation if the [[lithology]] is so variable that no single lithologic distinction is appropriate