Substratum

Noun
substratum (plural substrata)


 * 1) A layer that lies underneath another
 * 2)  The underlying cause or basis of something

Thesaurus
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

Etymology
sub, under + stratum, layer