A catalyst increases the rate or velocity of a chemical reaction without itself being changed in the overall process. Most biological catalysts are proteins called enzymes. The substance acted on by an enzyme is called a substrate. Enzymes speed up reactions by many orders of magnitude. For example, the enzyme catalase speeds up the conversion of hydrogen peroxide to water and oxygen by a factor of a billion.

True catalysts, such as enzymes, participate in the reaction, but are unchanged by it. Therefore, they can continue to catalyze subsequent reactions.

Catalysts change the rates of reactions, but do not affect the equilibrium of a reaction. That is, you cannot make more product from an enzyme-catalyzed reaction than you can from the same reaction without it. The enzyme simply helps to reach the equilibrium state faster than if it were not present.