Catalysis (/kəˈtælɪsɪs/, kə-TAL-iss-iss) is the increase in rate of a chemical reaction due to an added substance known as a catalyst^[1][2] (/ˈkætəlɪst/ KAT-əl-ist). Catalysts are not consumed by the reaction and remain unchanged after the reaction.^[3] If the reaction is rapid and the catalyst is recycled quickly, a very small amount of catalyst often suffices;^[4] 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.

The rate increase occurs because the catalyst allows the reaction to occur by an alternative mechanism which may be much faster than the noncatalyzed mechanism. However the noncatalyzed mechanism does remain possible, so that the total rate (catalyzed plus noncatalyzed) can only increase in the presence of the catalyst and never decrease.^[5]

Catalysis may be classified as either homogeneous, whose components are dispersed in the same phase (usually gaseous or liquid) as the reactant, or heterogeneous, whose components are not in the same phase. Enzymes and other biocatalysts are often considered as a third category.

Catalysis is nearly ubiquitous in chemical industry.^[6] Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.^[7]

The term "catalyst" is derived from Greek καταλύειν, kataluein, meaning "loosen" or "untie". The concept of catalysis was invented by chemist Elizabeth Fulhame, based on her novel work in oxidation-reduction experiments.^[8][9]