sub·stra·tum 
(sŭbstrā′təm, -străt′əm)
(sŭbstrā′təm, -străt′əm)n. pl. sub·stra·ta(-strā′tə, -străt′ə) or sub·stra·tums
1.
a. An underlying layer.
b. A layer of earth beneath the surface soil; subsoil.
2. A foundation or groundwork.
3. The material on which another material is coated or fabricated.
4. Philosophy The underlying characterless substance that supports attributes of material reality.
5. Biology A substrate.
6. Linguistics A substrate.
[New Latin substrātum, from neuter of Latin substrātus, past participle of substernere, to lay under : sub-, sub- + sternere, to stretch, spread; see ster-2 in the Appendix of Indo-European roots.]
sub·strative adj.