The Collaborative International Dictionary of English v.0.48:

Reformer \Re*form"er\ (r?*f?rm"?r), n.
   1. One who effects a reformation or amendment; one who labors
      for, or urges, reform; as, a reformer of manners, or of
      abuses.
      [1913 Webster]

   2. (Eccl.Hist.) One of those who commenced the reformation of
      religion in the sixteenth century, as Luther, Melanchthon,
      Zwingli, and Calvin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

reformer
    n 1: a disputant who advocates reform [syn: reformer,
         reformist, crusader, social reformer, meliorist]
    2: an apparatus that reforms the molecular structure of
       hydrocarbons to produce richer fuel; "a catalytic reformer"