What is a fitting that provides a liquid seal to prevent sewer gases from escaping called?

A fitting that provides a liquid seal to prevent sewer gases from escaping is known as a trap. Traps are essential components in plumbing systems, as they are designed to hold a small amount of water at all times. This water serves as a barrier, blocking foul-smelling gases from the sewer line from entering the living spaces of buildings.

Traps are installed in various locations, including under sinks, bathtubs, and floor drains, to ensure that the spaces remain free of harmful sewer fumes. The water in the trap acts as a seal, which is crucial for maintaining a healthy indoor environment.

In contrast, vents are pipes that allow air to enter the plumbing system to maintain proper pressure, but they do not provide a seal against gases. A seal might refer to various types of fittings, but it is not a specific term recognized in the context of plumbing traps. Caps are used to close the ends of pipes tightly but do not offer the same functionality in terms of preventing gas from escaping as traps do.