stratum

Stratum is a term used to denote a layer or bed of material within a stratified structure. The plural form is strata. The word is applied across several disciplines to refer to a distinct layer that differs in composition, texture, or properties from layers adjacent to it.

In geology and earth science, a stratum (or bed) is a volume of rock or soil with consistent characteristics that was deposited during a particular interval of time. Strata are studied in stratigraphy to reconstruct the sequence of geological events. The principle of superposition posits that, in undisturbed sequences, older strata lie beneath younger ones. Stratigraphic methods include lithostratigraphy (based on rock type), biostratigraphy (using fossils), and chronostratigraphy (linking layers to time). A stratigraphic column or section summarizes the arrangement of strata and can reveal unconformities that indicate missing intervals in deposition.

In archaeology, stratigraphy records the order of human activity at a site. Excavated layers are interpreted to establish a relative sequence of occupation, use, or deposition, aiding in dating artifacts and features.

In biology and anatomy, stratum denotes a layer of tissue. Examples include the epidermal strata of the skin (stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum) and layered regions in organs such as the hippocampus or retina. The term also appears in broader histological contexts to describe any layered organization.

In sociology, social strata refer to hierarchical layers within a society, typically defined by indicators such as income, occupation, education, or caste.

The term derives from Latin stratum, meaning a sheet or bed. Substratum and superstratum describe underlying and overlying layers, respectively.