A reagent /riˈeɪdʒənt/ is a substance or compound added to a system to cause a chemical reaction, or added to see if a reaction occurs. 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. Solvents, though involved in the reaction, are usually not called reactants. Similarly, catalysts are not consumed by the reaction, so are not reactants.