The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A bed or layer artificially made; a course.
      [1913 Webster]