Catalysis is the increase in rate of a chemical reaction due to an added substance known as a catalyst. Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quickly, very small amounts of catalyst often suffice; mixing, surface area, and temperature are important factors in reaction rate. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product, in the process of regenerating the catalyst.

जब किसी रासायनिक अभिक्रिया की गति किसी पदार्थ की उपस्थिति मात्र से बढ़ जाती है तो इसे उत्प्रेरण (Catalysis) कहते हैं। जिस पदार्थ की उपस्थिति से अभिक्रिया की गति बढ़ जाती है उसे उत्प्रेरक (catalyst) कहते हैं। उत्प्रेरक अभिक्रिया में भाग नहीं लेता, केवल क्रिया की गति को प्रभावित करता है।