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]

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "substratum":
   air, atom, atomic particles, band, base, basement, basis,
   bearing wall, bed, bedding, bedrock, belly, belt, bottom,
   bottom side, breech, brute matter, building block, buttocks,
   chemical element, component, constituent, core, couche, course,
   deck, downside, earth, element, elementary particle,
   elementary unit, fire, floor, flooring, fond, footing, foundation,
   fundament, fundamental, fundamental particle, gallery, ground,
   grounds, groundwork, hardpan, hyle, hypostasis, infrastructure,
   layer, ledge, level, lower side, lowest layer, lowest level,
   material, material world, materiality, matter, measures, meat,
   molecule, monad, natural world, nature, nether side,
   nethermost level, overlayer, overstory, pavement, physical world,
   plenum, principle, radical, riprap, rock bottom, root, rudiment,
   seam, seat, shelf, sill, solid ground, solid rock, stage, step,
   stereobate, story, stratum, stuff, stylobate, substance, substrate,
   substruction, substructure, superstratum, terra firma,
   the four elements, thickness, tier, topsoil, underbelly,
   underbuilding, undercarriage, undergirding, underlayer, underneath,
   underpinning, underside, understory, understratum, understruction,
   understructure, unit of being, water, zone