Other forms: retardants
A retardant is anything that prevents something from happening, or inhibits it from growing or spreading. Using a rust retardant on your car, for example, should help keep it from getting rusty.
A fire or flame retardant is a type of chemical that stops fire from starting or spreading. Sometimes fire retardants are sprayed or dumped on a fire to put it out, like the foam fire retardant you might spray from a hand-held fire extinguisher onto a grease fire in a kitchen. Retardant comes from the verb retard, "make slow, or make slower."