A reagent /riˈeɪdʒənt/ is a substance or compound added to a system to cause a chemical reaction, or added to test if a reaction occurs.[1] The terms reactant and reagent are often used interchangeably—however, a reactant is more specifically a substance consumed in the course of a chemical reaction.[1] Solvents, though involved in the reaction, are usually not called reactants. Similarly, catalysts are not consumed by the reaction, so they are not reactants. In biochemistry, especially in connection with enzyme-catalyzed reactions, the reactants are commonly called substrates.