What is the name of a piece of pipe less than 12 inches in length that is threaded on both sides?

A piece of pipe that is less than 12 inches in length and threaded on both ends is commonly known as a nipple. This type of fitting is specifically designed to connect two larger fittings or pipes. Nipples are usually short, allowing for easier installation in tight spaces or configurations where longer pieces of pipe would be cumbersome.

In contrast, a coupling is used to connect two pipes together and typically does not have threads on both sides. An elbow is a fitting used to change the direction of the flow in piping, featuring a bend rather than being straight. A reducer is utilized to connect pipes of different diameters and does not fit the description of having threads at both ends. Hence, the term "nipple" accurately captures the specifications described in the question.