Which term describes a unit of an electrical system that carries or controls electric energy as its principal function?

The term that describes a unit of an electrical system that carries or controls electric energy as its principal function is "Device." A device can encompass a variety of electrical apparatuses that perform specific tasks, such as switches, motors, relays, and appliances, where the fundamental purpose is to manipulate electrical energy in some way.

A conductor, on the other hand, is primarily related to materials that allow the flow of electric current, such as wires or cables, but it doesn’t control or carry energy independently as a device would. Insulators serve to prevent the flow of electricity, ensuring safety by separating conductors from unintended grounds or surfaces. Connectors are components that join electrical circuits together but don’t inherently manipulate or control energy; they simply facilitate connections between different parts. Therefore, the term "Device" accurately captures any component with a primary function of controlling or carrying electric energy in the system.