In plumbing and piping, a nipple is a fitting, consisting of a short piece of pipe, usually provided with a male pipe thread at each end, for connecting two other fittings.
Basic pattern of steel pipe nipples is a short piece of pipe with threads at both end or at one end. Its function is to connect pipes or other fittings in same or different diameter. We also call the basic type as barrel nipple or pipe nipple. It can be simply made from a piece of pipe.
Threads used on nipples are BSP, BSPT, NPT, NPSM and Metric.