What does "coupling" mean?

Coupling: (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. 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.