What defines a coupling in electrical fittings?

A coupling is defined as a fitting that serves the purpose of joining two lengths of conduit or raceway together. This function is essential in electrical installations, as it allows for the seamless transition between segments of wiring systems, ensuring that electrical conductors are protected and properly routed through the installed raceways.

By using couplings, electricians can extend conduits or raceways to reach the necessary lengths without compromising the system's integrity or safety. This connectivity is critical for maintaining a continuous pathway for electrical wiring, which is crucial in both residential and commercial installations.

The other options describe different components or functions that do not pertain to the specific role of a coupling. For example, while one option mentions terminating wireways, this describes a completely different fitting function, which is not related to the coupling's purpose.