In plumbing, what is the term for the vertical pipe that allows air to enter the drainage system?

The correct term for the vertical pipe that allows air to enter the drainage system is the vent stack. Vent stacks play a crucial role in plumbing systems, as they connect to the atmospheric pressure, thereby helping to maintain uniform pressure within the drainage pipes. This arrangement ensures that wastewater can flow freely without creating negative air pressure, which could lead to slow drainage or siphoning of traps, potentially allowing sewer gases to enter the building.

The other terms presented are related to plumbing but refer to different components. For instance, a sink drain is the pipe that carries wastewater away from the sink, while a riser is typically a vertical pipe that carries water upward within a plumbing system. A waste pipe is generally used to convey wastewater from fixtures to the drainage system but does not specifically serve the purpose of venting air. Thus, the vent stack is specifically designed for venting air and facilitating proper drainage.