What type of lighting is referred to as a tube?

The type of lighting referred to as a "tube" is fluorescent lighting. This terminology stems from the structure of fluorescent lamps, which typically come in long cylindrical shapes resembling tubes. These lamps operate by using electrical energy to excite mercury vapor, which emits ultraviolet light. This light then interacts with a phosphor coating on the inside of the tube, producing visible light.

Fluorescent lighting is widely used in commercial and industrial settings due to its efficiency and brightness, making it an economical choice for large spaces. The tubular form allows for easy installation in fixtures designed to hold these types of bulbs, enhancing versatility in lighting configurations.

In contrast, incandescent bulbs are usually rounded or bulbous and do not have a tubular form. LED lighting can come in various shapes and sizes, but the term "tube" is specifically associated with fluorescent fixtures. Compact fluorescent lamps (CFLs) do have a compact form but are not categorized as tubes—they are more of a spiral shape. Thus, fluorescent lighting is distinctly recognized and commonly referred to as a tube, aligning with the nature of the question.