n. The layer, or stratum, of earth on which the soil rests; the subsoil.
the GNU version of the Collaborative International Dictionary of English
n. The layer, or stratum, of earth on which the mold, or soil, rests; subsoil.
The Century Dictionary and Cyclopedia
n. A substratum; an underlying stratum; the stratum lying immediately beneath, or forming the lower portion of the one designated: not often used except figuratively.