What is the meaning of stratum?

noun

• 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. )
• People having the same social, economic, or educational status.
  • usage: "the working class"; "an emerging professional class"
  • synonyms: class, social class, socio-economic class
• An abstract place usually conceived as having depth.
  • usage: "a good actor communicates on several levels"; "a simile has at least two layers of meaning"; "the mind functions on many strata simultaneously"
  • synonyms: level, layer