coupler

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that couples, especially a device for coupling two railroad cars.
• noun A device connecting two organ keyboards so that they may be played together.

from The Century Dictionary.

• noun In zoology, a plate which joins one of the swimming-appendages of a crustacean to its fellow on the opposite side of the body.
• noun One who or that which couples, joins, or unites.

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

• noun One who couples; that which couples, as a link, ring, or shackle, to connect cars.
• noun a contrivance by which any two or more of the ranks of keys, or keys and pedals, are connected so as to act together when the organ is played.

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

• noun Someone who couples things together, especially someone whose job it is to couple railway carriages.
• noun Anything that serves to couple things together; but especially a device that couples railway carriages.
• noun music A device that connects two keyboards of an organ together so that they play together.
• noun A device used to convert electronic information into audible sound signals for transmission over telephone lines.
• noun An electrical device used to transfer energy from one electric device to another, especially without a physical connection.

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

• noun a mechanical device that serves to connect the ends of adjacent objects