Definition: stratum

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Source: WordNet (r) 1.7 

stratum
     n : one of several parallel layers of material arranged one on
         top of another (such as a layer of tissue or cells in an
         organism)

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.