A drain waste vent is used to allow drain water to flow freely by permitting air the enter the system, usually through a vent stack or an air admittance valve. Use with the [plumbing] tag

A drain waste vent is a component of a home's plumbing system that is responsible for allowing drain water to run freely by allowing air to enter the system, and thus prevent a siphon effect. Since sewers are filled with gases that range from unpleasant to dangerous, vents must either allow gasses out where they can dissipate (like a roof stack vent) or use a device like an air admittance valve (that allows air in, but not out)