substratum /ˈsʌbˌstreɪtəm/ noun plural substrata /-ˌstreɪtə/ /ˈsʌbˌstreɪtə/ plural substrata /-ˌstreɪtə/ /ˈsʌbˌstreɪtə/ Learner's definition of SUBSTRATUM
[count]
: a layer of something (such as soil or rock) that is under another layer(土壤或岩石等的)底层,下层 —
often used figuratively常用作比喻