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