Source : Webster's Revised Unabridged Dictionary (1913)

Reducer \Re*duc"er\, n.
   1. (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.

   2. (Photog.) A reducing agent, either a developer or an agent
      for reducing density.

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

Source : WordNet®

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 slackened silver [syn: {reducing
          agent}, {reductant}]
     2: pipefitting that joins two pipes of different diameter