What is termed a Fixture Drain in plumbing?

A fixture drain refers specifically to the drain that serves an individual plumbing fixture, such as a sink, toilet, or bathtub. When considering the answer, a single trap is crucial to the function of a fixture drain, as it is designed to prevent sewer gases from entering the living space by creating a water seal. Each fixture typically has its own trap, which is connected to its corresponding drain line.

While the other options may be related to plumbing systems, they do not accurately define what a fixture drain is. For instance, a system of pipes encompasses more than just a single fixture's drainage requirements and includes various interconnected parts of the plumbing setup. A type of vent relates to allowing air to enter the plumbing system to maintain proper pressure and prevent siphoning, rather than focusing on the physical drainage of the fixture itself. Lastly, a horizontal branch pipe refers to a section of pipe that conveys wastewater horizontally and may serve multiple fixtures but is not specific to the concept of a single fixture drain.

Therefore, the essence of a fixture drain is encapsulated in the function and designation of a single trap associated with each plumbing fixture.