Retarder /(?)/

Re·tard·er

Retarder

n.
  1. One who, or that which, retards.
  2. Any of various devices, as a helix of flat metal strip, introduced into a boiler tube to increase the heating effect of the fire. (Steam Boiler)
  3. A substance, as potassium bromide, added to a developer to retard its action. (Photog.)