Merriam-Webster Collegiate Dictionary - stratum

 noun  (plural strata)  Etymology: New Latin, from Latin, spread, bed, from neuter of stratus, past participle of sternere to spread out — more at strew  Date: 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 ~ 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 especially 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 occasionally 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. Strata may someday establish itself as a singular like agenda, but that use is still not established.