noun

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