A reagent is any substance used in a chemical reaction. It usually implies a chemical that is added in order to bring about the chemical change.
For example Hydrochloric acid is the chemical reagent that would cause Calcium carbonate to release Carbon dioxide. Similarly, but less obvious, Hydrochloric 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.
The word "reagent" has also been used in some fantasy literature, meaning a physical substance used as a component for a magic spell. This is an extension of the term's use in chemistry. The Ultima series of computer games are an example of this use.
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