An Atmospheric Vacuum Breaker (AVB) is a backflow prevention device used in plumbing to prevent backflow of non-potable liquids into the drinking water system.
It is usually constructed of brass and resembles a 90-degree elbow with a hood on its top to allow air to enter the water system if a siphon attempts to form.
Inside this elbow is a poppet valve that is held "up" by the water pressure found in the system, closing the air entrance to the device.
[1]These devices, since they work on atmospheric principles, cannot be installed in an enclosure containing air contaminants.
AVBs must be installed a minimum of 6 inches above the highest usage point in the system (i.e. sprinkler, drip emitter, etc.)