RETARDER (2)

Re‐tard″er (rē̍‐tär″dẽr), n. 1. 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. 2. (Photog.) A substance, as potassium bromide, added to a developer to retard its action.