(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 enzyme.
(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.
The term should be used with care. Either the context or a specific statement should always make it clear which chemical species in a reaction is regarded as the substrate.
