reducer

from The Century Dictionary.

• noun One who or that which reduces, in any sense.
• noun A joint-piece for connecting pipes of varying diameter. It may be of any form, straight, bent, etc. Also called reducing-coupling.
• noun In arithmetic, a multiplier used to reduce to another denomination.
• noun A device used in electric power-stations for lowering the voltage. It usually consists in alternating-current circuits of a step-down transformer and in direct-current circuits of a motor-generator.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who, or that which, reduces.
• noun 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.
• noun A reducing motion.
• noun A reducing valve.
• noun A hydraulic device for reducing pressure and hence increasing movement, used to transmit the load from the hydraulic support of the lower shackle to the lever weighing apparatus in some kinds of heavy testing machines.
• noun (Photog.) A reducing agent, either a developer or an agent for reducing density.
• noun (Chem.) a reducing agent.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Something that reduces.
• noun photography A liquid used in the process of developing a negative to reduce its darkness.
• noun plumbing A component used to connect two pipes of different bores.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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
• noun pipefitting that joins two pipes of different diameter