Definition of substratum noun from the Oxford Advanced American Dictionary

 

substratum

 noun
noun
NAmE//ˈsʌbˌstreɪt̮əm//
 
, NAmE//ˈsʌbˌstræt̮əm//
 
(pl. substrata
NAmE//ˈsʌbˌstreɪt̮ə//
 
, NAmE//ˈsʌbˌstræt̮ə//
 
)
(technology)
 
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a layer of something, especially rock or soil, that is below another layer a substratum of clay the substrata of society
See the Oxford Advanced Learner's Dictionary entry: substratum