Etymology: New Latin, from Latin, spread, bed, from neuter of stratus, past participle of sternere to spread out — more at strew
1. a bed or layer artificially made
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 stratum 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.
New Collegiate Dictionary. 2001.