Which of the following best describes a catalyst?

A catalyst is best defined as a substance that increases the rate of a chemical reaction without being consumed in the process. It works by lowering the activation energy needed for the reaction to occur, allowing reactants to convert into products more quickly. While a catalyst may participate in the reaction temporarily, it will regenerate its original state, allowing it to facilitate further reactions.

The other options do not accurately capture the definition of a catalyst. For instance, a substance that is consumed in a reaction would imply that it is a reactant, not a catalyst. A reaction that cannot be reversed describes a type of reaction (irreversible reactions) rather than any substance involved in the reaction. Lastly, a product of a reaction refers to the outcome of a chemical change, not a substance that speeds up or facilitates the reaction itself.