substratum

US [ˈsʌbˌstrɑtəm]
UK [ˈsʌbˌstrɑːtəm]
  • n.(尤指岩石或土壤的)下卧层;底土层
  • Web下层;底层;基质
Plural Form:substrata  
substratum
n.
1.
(尤指岩石或土壤的)下卧层;底土层a layer of sth, especially rock or soil, that is below another layer