The Collaborative International Dictionary of English v.0.48:

Reducing agent \Re*du"cing a`gent\, n. (Chem.)
   a substance that causes reduction of another substance in a
   chemical reaction, as by donating electrons or adding
   hydrogen atoms; as, lithium hydride is a powerful reducing
   agent.
   [PJC]

WordNet (r) 3.0 (2006):

reducing agent
    n 1: a substance capable of bringing about the reduction of
         another substance as it itself is oxidized; used in
         photography to lessen the density of a negative or print by
         oxidizing some of the loose silver [syn: reducing agent,
         reducer, reductant]