What does "reducing" mean?

Reducing: a & n. from Reduce. Reducing furnace (Metal.), a furnace for reducing ores. -- Reducing pipe fitting, a pipe fitting, as a coupling, an elbow, a tee, etc., for connecting a large pipe with a smaller one. -- Reducing valve, a device for automatically maintaining a diminished pressure of steam, air, gas, etc., in a pipe, or other receiver, which is fed from a boiler or pipe in which the pressure is higher than is desired in the receiver.