The undesirable reversal of flow of water or mixtures of water and other liquids, gases or other substances into the distribution pipes of the potable supply of water from any source or sources. This can occur through backpressure or backsiphonage.

Backpressure – Any elevation of pressure in the downstream piping system (by pump, elevation of piping, steam pressure, air pressure, etc.) above the supply pressure at the point of consideration, which would cause or tend to cause a reversal of the normal direction of flow.

Backsiphonage – A form of backflow due to a reduction in system pressure, which causes a sub-atmospheric pressure to exist in the water system.