What is defined as a rigid conduit made of materials like iron, steel, or plastic?

The term that describes a rigid conduit made from materials such as iron, steel, or plastic is pipe. In mechanical and plumbing contexts, "pipe" refers specifically to a tubular conduit used to convey fluids or gases, and it is typically characterized by its structural integrity and rigidity, which makes it suitable for carrying various substances under different pressures.

In the context of the options provided, "tubing" usually refers to more flexible structures with thinner walls, often used for lighter applications, while "channel" typically describes a shape that provides structural support rather than a conduit for flow. "Casing," meanwhile, is commonly associated with enclosing or housing systems rather than serving as a conduit itself. Therefore, pipe is the most accurate term for a rigid conduit made from the specified materials.