The Collaborative International Dictionary of English v.0.48:

Reagent \Re*a"gent\ (r[-e]*[=a]"jent), n. (Chem.)
   A substance capable of producing with another a reaction,
   especially when employed to detect the presence of other
   bodies; a test.
   [1913 Webster]

WordNet (r) 3.0 (2006):

reagent
    n 1: a chemical agent for use in chemical reactions

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "reagent":
   acid, acidity, agent, alkali, alkalinity, alloisomer, anion,
   antacid, atom, base, biochemical, cation, chemical,
   chemical element, chromoisomer, compound, copolymer, dimer,
   element, heavy chemicals, high polymer, homopolymer, hydracid,
   inorganic chemical, ion, isomer, macromolecule, metamer, molecule,
   monomer, neutralizer, nonacid, organic chemical, oxyacid, polymer,
   pseudoisomer, radical, sulfacid, trimer