Substratum

n. layer that lies beneath another; material that serves as the foundation for something

In linguistics, a substratum (lat. sub: under + stratum: layer → lower layer) is a language which influences another one while that second language supplants it. The term is also used of substrate interference, i.e. the influence exerted by the substratum language on the supplanting language. According to some classifications, this is one of three main types of linguistic interference: substratum interference differs from both adstratum, which involves mutual borrowing between languages of roughly equal prestige and no language replacement, and superstratum, which refers to the influence a socially dominating language has on another, receding language which might be eventually relegated to the status of a substratum language.
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Noun
1. any stratum lying underneath another
(synonym) substrate
(hypernym) stratum

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

(n.)
That which is laid or spread under; that which underlies something, as a layer of earth lying under another; specifically (Agric.), the subsoil.
  

Synonyms and related words:
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