In plumbing and piping, a nipple is a connector, consisting of a small piece of pipe, usually with a male thread at each end, to connect two other fittings. The threads used on the nipples are BSP, BSPT, NPT, NPSM and Metric. When it meets high hygiene standards, it can also be regarded as a kind of sanitary fittings.
The length of the nipple is usually specified by the total length with wires. It can have a hexagonal section in the center to grasp the key (sometimes called “hexagonal nipple”), or it may simply be made from a short piece of pipe (sometimes referred to as a “barrel nipple” or “pipe nipple”). A “close nipple” has no unthreaded area; when screwed firmly between two female connectors, a very small portion of the nipple remains exposed. A close nipple can only be unscrewed by grasping a threaded end with a pipe wrench which will damage the threads and necessitate replacing the nipple, or by using a specialty tool known as a nipple wrench (or known as an internal pipe wrench) which grips the inside of the pipe, leaving the threads undamaged. When the ends are of two different sizes, it is called a reducer or unequal nipple.