In plumbing, what is a "trap"?

A trap in plumbing is specifically designed to prevent sewer gases from entering buildings while still allowing wastewater to flow out. This is achieved through the design of the trap, which holds a small amount of water, creating a seal between the interior of the building and the sewer system. This water barrier effectively blocks harmful gases, such as methane and hydrogen sulfide, from drifting back into living spaces, which is crucial for maintaining indoor air quality and safety.

The functionality of a trap is essential in plumbing systems, as it serves both a practical and health-related purpose. The other options, such as devices for increasing water flow, filters for water purification, or valves for regulating water pressure, do not fulfill this critical role of gas containment and are thus not applicable to the definition of a trap in plumbing.