What term describes a nipple that is threaded on the ends and has a short unthreaded pipe in the middle?

The correct term for a nipple that is threaded on the ends with a short unthreaded pipe in the middle is referred to as a shoulder nipple. This type of nipple is designed with a stepped shoulder that allows for a snug fit against other components, providing support and maintaining a secure connection in a plumbing or piping system. The unthreaded middle section is crucial because it can help reduce turbulence and allow for fluid flow without restrictions.

The other terms listed refer to different types of fittings used in plumbing. An adapter nipple would typically connect two pipes of different sizes or threads, a coupling nipple is used to connect two pieces of pipe, and a reducing nipple serves to connect pipes of different diameters, often reducing the size of the connection. Each of these serves specific purposes in piping systems and does not have the distinct structure or function of a shoulder nipple.