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. ]