/ ˈsʌbstrɑːtəm; NAmE ˈsʌbstreɪtəm/ noun
( pl. sub·strata / ˈsʌbstrɑːtə; NAmE ˈsʌbstreɪtə/) ( technical ) a layer of sth, especially rock or soil, that is below another layer
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WORD ORIGIN
mid 17th cent.: modern Latin , neuter past participle (used as a noun) of Latin substernere , from sub- below + sternere strew. Compare with stratum .