High Quality Content by WIKIPEDIA articles! A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of a chemical reaction".Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants.