What is the name for the inside bottom elevation of a sewer line or drain pipe?

The correct term for the inside bottom elevation of a sewer line or drain pipe is "invert." This term refers specifically to the lowest point of the interior of the pipe, where fluids flow. Understanding the concept of invert is crucial in plumbing design and drainage systems, as it plays a significant role in ensuring proper flow and preventing blockages. The invert level helps in calculating slopes, which are essential to facilitate gravity drainage, ensuring that wastewater moves efficiently through the system.

The other options do not accurately describe this specific feature of sewer lines or drain pipes; they might refer to different aspects or components within plumbing systems but not the interior bottom elevation itself.