1. The chemical entity whose conversion to a product or products is catalysed by one or several enzymes.
  2. A solution or dry mixture containing all ingredients which are necessary for the growth of a microbial culture or for product formation.
  3. Component in the nutrient medium, supplying the organisms with carbon (C-substrate), nitrogen (N-substrate), etc.
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 166 (