What is the meaning of substratum?

noun

• A surface on which an organism grows or is attached.
  • usage: "the gardener talked about the proper substrate for acid-loving plants"
  • synonyms: substrate
• Any stratum or layer lying underneath another.
  • synonyms: substrate
• An indigenous language that contributes features to the language of an invading people who impose their language on the indigenous population.
  • usage: "the Celtic languages of Britain are a substrate for English"
  • synonyms: substrate