Hypertext Webster Gateway: "substratum"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
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.
From WordNet (r) 1.7 (wn)
substratum
n : any stratum lying underneath another [syn: {substrate}]
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