reagent

n. substance used to detect or measure another substance or to convert one substance into another by chemical reaction (Chemistry)

A reactant or reagent is a substance consumed during a chemical reaction. Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants.Although the terms reactant and reagent are often used interchangeably, reagent is often used in a more specialized sense as "a test substance that is added to a system in order to bring about a reaction or to see whether a reaction occurs". Examples of such analytical reagents include Fehling's reagent and Tollens' reagent. In organic chemistry, reagents are compounds or mixtures, usually composed of inorganic or small organic molecules, that are used to effect a transformation on an organic substrate. Examples of organic reagents include the Collins reagent, Fenton's reagent, and Grignard reagent.
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(n.)
A substance capable of producing with another a reaction, especially when employed to detect the presence of other bodies; a test.
  

A substance used to produce a chemical reaction to detect, measure, produce, etc. other substances.

A substance or mixture that is useful in chemical analysis or synthesis.