Any chemical or other substance that increases the rate at which a chemical reaction takes place without itself becoming permanently altered by the reaction. Platinum is one of the most effective catalysts. In printing inks, driers are added as catalysts, as they facilitate the chemical reactions (such as oxidation and/or polymerization) that cause an ink to dry. A substance which increases the rate of reaction but which is itself consumed or altered by the reaction is an intermediate. The opposite of a catalyst is an inhibitor.