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.]