1. 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
2. pipefitting that joins two pipes of different diameter
3. One who, or that which, reduces.
4. 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.