Illustrated Glossary of Organic Chemistry

Substrate: The starting material (other than enzyme or coenzyme) for an enzymatic chemical reaction. Sometimes the term also means the entity that is attacked in a chemical reaction or mechanism step; this usage should be avoided because there is no strict method for identifying an "aggressor molecule" and a "victim molecule".

In this enzyme reaction, ethanol is the substrate, NAD⁺ is the coenzyme which oxidizes ethanol, alcohol dehydrogenase is the enzyme, and acetaldehyde is the product.