reducer(n = noun.substance) reducing agent, reductant - a substance capable of bringing about the reduction of another substance as it itself is oxidized; used in photography to lessen the density of a negative or print by oxidizing some of the loose silver;
A contrivance for reducing the dimensions of one part so as to fit it to another, as a reducing coupling, or a device for holding a drilling a chuck. [Webster 1913 Suppl.]
A reducing agent, either a developer or an agent for reducing density. [Webster 1913 Suppl.]