\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.

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