Meaning

noun a surface on which an organism grows or is attached

noun any stratum or layer lying underneath another

noun an indigenous language that contributes features to the language of an invading people who impose their language on the indigenous population

originLearned borrowing from New Latin substrātum, nominalisation of the neuter of Latin substrātus, perfect passive participle of substernō. By surface analysis, sub- + stratum.

Etymology from Wiktionary, CC BY-SA 3.0