n.
foundation, base, basis, footing, ground, groundwork, root, backdrop, backbone, bed, bottom, corner stone, fortification, fundament, ground form, hard pan, keystone, pedestal, socle, substratum, substructure, grass roots
adj.
basic, bread and butter, fundamental, primary, principal, underlying, elementary, basal, abecedarian, constitutive, elemental, guiding, parent, rudimental, rudimentary, staple, working