Source : Webster's Revised Unabridged Dictionary (1913)

Substratum \Sub*stra"tum\, n.; pl. {Substrata}. [L. substratus,
   p. p. of substernere to strew under; sub under + sternere to
   strew. See {Stratum}.]
   1. That which is laid or spread under; that which underlies
      something, as a layer of earth lying under another;
      specifically (Agric.), the subsoil.

   2. (Metaph.) The permanent subject of qualities or cause of
      phenomena; substance.

Source : WordNet®

substratum
     n : any stratum lying underneath another [syn: {substrate}]
     [also: {substrasta} (pl)]