Backflow can be described as "a reversal of the normal direction of flow within a piping system or as "the flow of water or other liquids, mixtures or substances into the distribution pipes of a potable water supply from any source other then the intended source of the potable water supply.

The potential for a backflow condition occurring in a water system is all too likely in many of our homes, factories and public buildings. The existence of improper plumbing connections presents cross-connections that may, under backflow conditions, permit the water to flow the wrong way within the piping.

The probability of backflow taking place at any given outlet may actually be very small. But in view of the large number of service connections, the multiple cross-connections at each service connection and the potential for cross-connections to be created, than the probability becomes very significant and must be dealt with in a positive manner.

A cross-connection can be described as “any arrangement whereby backflow can occur”, or as “any arrangement of pipes, fittings, fixtures, or devices that connects a non-potable water system to a potable water system”. It is the point at which a water-using fixture is connected to the water user's potable water system.

An unprotected cross-connection provides the path or route through which backflow can occur.
It can be: