Final answer:
An air admittance valve is a device that allows air to enter the plumbing system to balance pressure and prevent negative pressure from disrupting water seals in traps.
Step-by-step explanation:
An air admittance valve is a one-way valve designed to allow air to enter the plumbing drainage system when negative pressure develops in the piping system. These valves help to maintain the water seals in the traps, preventing sewer gases from entering the building and allowing for proper drainage. They react to the pressure differences caused by fluid movement within the system, similar in principle to the entrainment devices like the aspirator or Venturi constriction, which use fluid speed to create low pressures.