(1) A chemical species of particular interest, the reaction of which with some other chemical reagent is under observation (e.g. a compound that is transformed under the influence of a catalyst).
(2) The chemical entity whose conversion to a product or products is catalyzed by an enzymes.
(3) A solution or dry mixture containing all ingredients which are necessary for the growth of a microbial culture or for product formation.
(4) A component in the nutrient medium, supplying the organisms with carbon (C-substrate), nitrogen (N-substrate) etc.
