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A coupling (or coupler) (used in piping or plumbing) is a very short length of pipe or tube, with a socket at one or both ends that allows two pipes or tubes to be joined, welded (steel), brazed or soldered (copper, brass etc.) together.
Alternatively it is a short length of pipe with two female National pipe threads (NPT) (in North American terms, a coupler is a double female while a nipple is double male) or two male or female British standard pipe threads.
If the two ends of a coupling are different (e.g. one BSP threaded and one NPT threaded), then it is usually referred to as an adapter. Another variation is one plain socket and one threaded socket. Yet another variation would be 3/4" NPT to 1/2" NPT.
When the two ends use the same connection method but are of a different size, the terms reducing coupling or reducer are used.