Final answer:
A catalyst is a substance that increases the rate of a chemical reaction without being consumed, by lowering the activation energy and providing an alternative pathway for the reaction.
Step-by-step explanation:
A catalyst is a substance that increases the rate of a chemical reaction by reducing the amount of energy needed for the reaction to proceed (activation energy), without being consumed in the reaction itself. Catalysts function by providing an alternative pathway for the reaction to occur, which requires less energy.
Unlike reactants, a catalyst is not used up during the reaction and can be recovered unchanged at the end of the reaction. This allows a catalyst to be used repeatedly in a series of reactions. Catalysts are essential in many industrial processes and in biological systems where they are known as enzymes. They do not affect the equilibrium of a reaction, but they do help the system to reach equilibrium more quickly.