Source : Webster's Revised Unabridged Dictionary (1913)

Stratum \Stra"tum\, n.; pl. E. {Stratums}, L. {Strata}. The
   latter is more common. [L., from sternere, stratum, to
   spread; akin to Gr. ? to spread, strew. See {Strew}, and cf.
   {Consternation}, {Estrade}, {Prostrate}, {Stratus},
   {Street}.]
   1. (Geol.) A bed of earth or rock of one kind, formed by
      natural causes, and consisting usually of a series of
      layers, which form a rock as it lies between beds of other
      kinds. Also used figuratively.

   2. A bed or layer artificially made; a course.

Source : WordNet®

stratum
     n 1: one of several parallel layers of material arranged one on
          top of another (such as a layer of tissue or cells in an
          organism)
     2: an abstract place usually conceived as having depth; "a good
        actor communicates on several levels"; "a simile has at
        least two layers of meaning"; "the mind functions on many
        strata simultaneously" [syn: {level}, {layer}]
     [also: {strata} (pl)]