What is a piece of black steel pipe with a diameter of ⅛" to 2" called?

A piece of black steel pipe with a diameter of ⅛" to 2" is referred to as a nipple. Nipples are short lengths of pipe, typically threaded at both ends, used to connect two fittings in plumbing systems. Their small diameter allows them to fit into tight spaces, making them an essential component for creating connections between larger pipes or for connecting fixtures.

In plumbing, the term "nipple" specifically relates to this function, which aligns with the typical usage of black steel pipe in various plumbing applications. Black steel pipes are commonly used for gas lines and water piping due to their durability and resistance to corrosion.

The other options denote different plumbing fittings, but they don’t accurately describe the specific type of component referenced in the question. Couplings are used to join two pipe ends without significant length, elbows are fittings that change the direction of the piping, and caps are used to cover the end of a pipe. Each of these has a distinct purpose that differs from that of a nipple.