The Collaborative International Dictionary of English v.0.48:

Coupling \Coup"ling\ (-l?ng), n.
   1. The act of bringing or coming together; connection; sexual
      union.
      [1913 Webster]

   2. (Mach.) A device or contrivance which serves to couple or
      connect adjacent parts or objects; as, a belt coupling,
      which connects the ends of a belt; a car coupling, which
      connects the cars in a train; a shaft coupling, which
      connects the ends of shafts.
      [1913 Webster]

   Box coupling, Chain coupling. See under Box, Chain.

   Coupling box, a coupling shaped like a journal box, for
      clamping together the ends of two shafts, so that they may
      revolve together.

   Coupling pin, a pin or bolt used in coupling or joining
      together railroad cars, etc.
      [1913 Webster]