The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Metaph.) The permanent subject of qualities or cause of
      phenomena; substance.
      [1913 Webster]