Reagent
A reagent is any substance used in a chemical reaction. It usually implies a chemical that is added in order to bring about the chamical change.For example Hydrochloric acid is the chemical reagent that would cause Calcium carbonate to release Carbon dioxide. Similarly, but less obvious, Hydrocloric acid is the chemical reagent that reacts with Zinc to produce Hydrogen even though in this case the hydrogen comes from the acid and not the metal.
To classify any of the chemicals involved in a chemical reaction as the "Reagent" is largely a matter of convention or perspective.