The Collaborative International Dictionary of English v.0.48:

Retarder \Re*tard"er\, n.
   1. One who, or that which, retards.
      [1913 Webster]

   2. (Steam Boiler) Any of various devices, as a helix of flat
      metal strip, introduced into a boiler tube to increase the
      heating effect of the fire.
      [Webster 1913 Suppl.]

   3. (Photog.) A substance, as potassium bromide, added to a
      developer to retard its action.
      [Webster 1913 Suppl.]