What is the term for the fall (pitch) of a pipe in relation to a horizontal plane?

The term for the fall or pitch of a pipe in relation to a horizontal plane is "slope." In plumbing, slope is critical because it ensures proper drainage. A well-designed slope allows wastewater and sewage to flow efficiently through the pipe without stagnation. The recommended slope varies depending on the diameter of the pipe, but it is necessary to maintain it for effective drainage.

While "declination," "angle," and "elevation" might seem relevant in certain contexts, they do not specifically refer to the gradient or downward incline of pipes in plumbing. Declination generally refers to a downward inclination in a more abstract sense, and angle can refer to any specific measurement of inclination relative to a straight line but does not directly relate to the concept of drainage in plumbing. Elevation is used to describe the height or vertical position of an object or point rather than its tilt or slope. Thus, "slope" specifically captures the essence of how a pipe is angled downhill to facilitate proper flow.