Retarder

noun

1. One who retards; that which serves as a hindrance, impediment, or cause of retardation.
2. An obstruction, usually in the form of a twisted piece of sheet-metal, placed in a boiler-flue to retard the hot gases and make them give up their heat to the water in the boiler.
3. A device, such as a dash-pot, for checking the too rapid motion of any mechanism.
4. One who, or that which, retards.
5. Any of various devices, as a helix of flat metal strip, introduced into a boiler tube to increase the heating effect of the fire.