Reagent n. : A substance capable of producing with another a reaction, especially when employed to detect the presence of other bodies; a test.

Example:
How do you determine which Reagent is the best choice to prepare a buffer with a given pH?

Taxonomy
reagent, reaction, reactant, chemical reaction, free encyclopedia, compound, test substance, organic reagents, organic chemistry, search, Sulfur, interchangeably, Purity standards, presence, Fehling's reagent, Tollens' reagent, mixtures, organic molecules.

Defination from the web:
reagents are compounds or mixtures, usually composed of inorganic or small organic molecules, that are used to affect a ... For instance, reagent-quality water must have very low levels of impurities like sodium and chloride ions, silica, and bacteria, as well as a ...