STRATUM

noun (plural strata) see: strew Date: 1599 a bed or layer artificially made, 2. a sheetlike mass of sedimentary rock or earth of one kind lying between beds of other kinds, a region of the sea or atmosphere that is analogous to a ~ of the earth, a layer of tissue , a layer in which archaeological material (as artifacts, skeletons, and dwelling remains) is found on excavation, 3. a part of a historical or sociological series representing a period or a stage of development, a socioeconomic level of society comprising persons of the same or similar status especially with regard to education or culture, one of a series of layers, levels, or gradations in an ordered system , a statistical subpopulation, Usage: The plural strata has occasionally been used as a singular since the 18th century and is sometimes given the plural stratas . Current evidence shows senses 2, 3b, and 4 so used, with 3b the most common. Singular strata is persistent but not frequent. Strata may someday establish itself as a singular like agenda, but that use is still not established.