Updated June 12, 2014. Definition: A substrate is the medium in which a chemical reaction takes place or the reagent in a reaction that provides a surface for absorption. In biochemistry, an enzyme substrate is the substance the enzyme acts upon. Sometimes the word substrate is also used as a synonym for the reactant, which is the molecule consumed in a chemical reaction. Substrate (biology): In biology, the substrate may be the surface on which an organism grows or is attached. For example, a microbiological medium may be considered a substrate. The substrate may also be the material at the bottom of a habitat, such as gravel at the base of an aquarium. Substrate may also refer to the surface on which an organism moves. Substrate (materials science): In this context, a substrate is a base on which a process occurs. For example, if gold is electroplated over silver, the silver is the substrate. Substrate (geology): In geology, substrate is underlying stratum.