substratum
Concise Oxford English Dictionary © 2008 Oxford University Press:
substratum /sʌbˈstrɑːtəm, -ˈstreɪtəm/
▶noun (pl. substrata)
- 1 an underlying layer or substance, in particular a layer of rock or soil beneath the surface of the ground.
- 2 a foundation or basis.
'substratum' also found in these Oxford entries:

