coupler

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. One who or that which couples, joins, or unites. Specifically— In organbuilding, a mechanical contrivance by which the keys of one keyboard are so connected with corresponding keys of another that when the former are depressed the latter are also depressed, and thus both can be played by a single motion. Manual couplers connect manual keyboards with each other; pedal couplers connect the pedal keyboard to a manual. Unison couplers connect keys of the same pitch; octave couplers (sometimes loosely called super-octave or sub-octave) connect keys an octave apart. Octave couplers are sometimes arranged between the keys of a single keyboard, so that it may be coupled with itself. Couplers operate in only one direction; that is, the second keyboard may be coupled with the first, but not the reverse. Also copula.
2. n. In zoology, a plate which joins one of the swimming-appendages of a crustacean to its fellow on the opposite side of the body.

Wiktionary

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

GNU Webster's 1913

1. n. One who couples; that which couples, as a link, ring, or shackle, to connect cars.

WordNet 3.0

1. n. a mechanical device that serves to connect the ends of adjacent objects