What is a horizontal pipe defined as in plumbing?

In plumbing, a horizontal pipe is defined as a pipe or fitting that makes an angle of less than 45 degrees with the horizontal. This definition is crucial for ensuring proper drainage and waste flow in plumbing systems. When a pipe is installed at a slight incline (less than 45 degrees), it allows for the effective movement of fluids due to gravity, minimizing the chances of blockage and facilitating proper function within the plumbing infrastructure.

The other choices do not accurately reflect this definition. A pipe that runs vertically is the opposite of horizontal, and a literal definition of a level pipe does not provide the necessary context for understanding the technical requirements for plumbing installations. When considering fittings, while the angle may be less than 45 degrees, it's essential to focus on the overall characteristic of the pipe's orientation to properly classify it as horizontal.