One of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock) WordNet 3.0 & Open English WordNet

In geology and related fields, a stratum is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as either bedding surfaces or bedding planes. Wikipedia

A layer of sedimentary rock or sediment. Wikipedia Disambiguation

Layer of sedimentary rock or soil with internally consistent characteristics Wikidata

A layer of sedimentary rock having approximately the same composition throughout. Wiktionary

Layer of sedimentary rock having approximately the same composition throughout. Wiktionary (translation)