reducer

reducer
    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}]
    2: pipefitting that joins two pipes of different diameter
    

Reducer \Re*du"cer\ (-s?r), n.
   1. One who, or that which, reduces.
      [1913 Webster]

   2. (Mach.)
      (a) A contrivance for reducing the dimensions of one part
          so as to fit it to another, as a reducing coupling, or
          a device for holding a drilling a chuck.
      (b) A reducing motion.
      (c) A reducing valve.
      (d) A hydraulic device for reducing pressure and hence
          increasing movement, used to transmit the load from
          the hydraulic support of the lower shackle to the
          lever weighing apparatus in some kinds of heavy
          testing machines.
          [Webster 1913 Suppl.]

   3. (Photog.) A reducing agent, either a developer or an agent
      for reducing density.
      [Webster 1913 Suppl.]

   4. (Chem.) a reducing agent.
      [PJC]