Merriam-Webster Collegiate Dictionary - substratum

 noun  (plural substrata)  Etymology: Medieval Latin, from Latin, neuter of substratus, past participle of substernere to spread under, from sub- + sternere to spread — more at strew  Date: 1631 an underlying support ; foundation: as  a. substance that is a permanent subject of qualities or phenomena  b. the material of which something is made and from which it derives its special qualities  c. a layer beneath the surface soil; specifically subsoil  d. substrate 2