A substance which, in small amounts, influences a chemical reaction without chemically changing it. A substance that causes a change in the rate of a chemical reaction without itself being consumed by the reaction.

Catalysts, which work by changing a reaction's activation energy, or minimum energy needed for the reaction to occur, are used in numerous industrial processes. Substances that increase the reaction rate are called positive catalysts or simply catalysts, whereas substances that decrease the reaction rate are called negative catalysts, or inhibitors.

The presence of a small amount of an acid or base may catalyze some reactions. Finely divided metals (e.g. platinum, copper, iron, palladium, rhodium) or metal oxides (e.g. silicon dioxide, vanadium oxide) may also serve as catalysts. Biological catalysts are called Enzymes.