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]

WordNet (r) 3.0 (2006):

substratum
    n 1: a surface on which an organism grows or is attached; "the
         gardener talked about the proper substrate for acid-loving
         plants" [syn: substrate, substratum]
    2: any stratum or layer lying underneath another [syn:
       substrate, substratum]
    3: an indigenous language that contributes features to the
       language of an invading people who impose their language on
       the indigenous population; "the Celtic languages of Britain
       are a substrate for English" [syn: substrate, substratum]