Chemical reactions that take place in the presence of a catalyst are called catalytic reactions. In a chemical reaction, some of the original chemical bonds of the reacting molecules must dissociate and form new ones, which requires a certain activation energy. In some systems where chemical reactions are difficult to occur, the addition of a third substance (catalyst) that helps rearrange the chemical bonds of the reacting molecules acts to reduce the activation energy of the reaction, thus accelerating the chemical reaction and controlling the selectivity and steric regularity of the products.

Definition: According to the definition proposed by IUPAC in 1981, a catalyst is a substance that accelerates the rate of a reaction without changing the standard Gibbs free enthalpy change of that reaction. This effect is called catalysis. A screening involving a catalyst is a catalytic reaction.