An Air Gap is a physical separation between the free flowing discharge end of a potable water supply pipeline and an open or non-pressure receiving vessel. An approved air gap shall be at least double the diameter of the supply pipe measured vertically above the overflow rim of the vessel and in no case should be less than 1 inch.
Unlike other backflow prevention assemblies, an air gap is a totaly non-mechanical means of backflow prevention.
An Air Gap is considered the maximum protection available against back pressure backflow or back-siphonage, but unfortunately is not always practical and can easily be bypassed.
The general configuration of an approved Air Gap is illustrated here: