Final answer:
A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative pathway with lower activation energy, and it remains unchanged at the end of the reaction.
Step-by-step explanation:
A catalyst is a substance that is added to a chemical reaction to increase the rate of the reaction without being consumed by it. Catalysts are not reactants or products because they don't undergo a change during the reaction process. They function by providing an alternative reaction pathway with a lower activation energy, effectively increasing the number of successful particle collisions per unit time and helping the reaction to achieve equilibrium faster.