Air gap

"An air gap" means a visible, unobstructed and complete physical air break between the lowest level of water discharge and the level of potentially contaminated fluid downstream (critical water level) within a cistern, vessel, fitting or appliance, hereinafter called a receptacle, that: 

a. is not less than 20 mm or twice the internal diameter of the inlet pipe whichever is the greater; and 

b. from which water discharges at not more than 15° from the vertical centreline of the water stream. 

Follow the links below to get definitions of each of the recognised backflow prevention arrangements.