rə̇ˈtärdər, rēˈ-, -tȧdə(r noun
( -s )
Etymology: retard (I) + -er
: one that retards: as
a. : a substance that when added to a cement or to gypsum plaster prolongs the setting time
b. : restrainer
c. : a power-operated braking device placed in the classification tracks of hump yards to control the speed of moving railroad cars
d. : a substance added in small proportion to a rubber compound for retarding vulcanization