D defs.my
Entry 3 senses Webster, 1913

Retarder

· Re·tard·er · IPA
01 n. One who, or that which, retards.
  1. 1.
    One who, or that which, retards.
  2. 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. 3.
    A substance, as potassium bromide, added to a developer to retard its action.(Photog.)