Pressure created by any means in the water distribution system, which by being in excess of the pressure in the water supply mains, causes a potential backflow condition.
A pressure less than or equal to 4.33 psi (29.88 kPa) or the pressure exerted by a 10-foot (3048 mm) column of water.
The backflow of potentially contaminated water into the potable water system as a result of the pressure in the potable water system falling below atmospheric pressure of the plumbing fixtures, pools, tanks or vats connected to the potable water system distribution piping.
A reversal in the drainage system.
The flow of water or other liquids, mixtures or substances into the distribution pipes of a potable water supply from any source except the intended source.
(International Plumbing Code® 202.0)