Source : Webster's Revised Unabridged Dictionary (1913)

Retarder \Re*tard"er\, n.
   One who, or that which, retards.

Retarder \Re*tard"er\ (r[-e]*t[aum]r"d[~e]r), n.
   1. (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.

   2. (Photog.) A substance, as potassium bromide, added to a
      developer to retard its action.