[stra.tum] n, pl stra.ta [NL, fr. L, spread, bed, fr. neut. of stratus, pp. of sternere to spread out--more at strew] (1599) 1: a bed or layer artificially made
2. a: a sheetlike mass of sedimentary rock or earth of one kind lying between beds of other kinds b: a region of the sea or atmosphere that is analogous to a stratum of the earth c: a layer of tissue "deep ~ of the skin" d: a layer in which archaeological material (as artifacts, skeletons, and dwelling remains) is found on excavation 3 a: a part of a historical or sociological series representing a period or a stage of development b: a socioeconomic level of society comprising persons of the same or similar status esp. with regard to education or culture
4: one of a series of layers, levels, or gradations in an ordered system "strata of thought"
5: a statistical subpopulation usage The plural strata has occas. been used as a singular since the 18th century and is sometimes given the plural stratas "there was a strata of Paris which mere criticism of books fails to get hold of --Ezra Pound" "a Roman burial ground suggests stratas of corruption and decay --Connie Fletcher, Booklist". Current evidence shows senses 2, 3b, and
4. so used, with 3b the most common. Singular strata is persistent but not frequent. It may someday establish itself as a singular like agenda, but its current status is something less than established.