A Cross-Connection is an actual or potential link between the potable water supply and a non-potable source. A cross-connection can be avoided by eliminating the link between the two sources. If the link cannot be removed, then the potable water supply must be protected through the use of a backflow prevention assembly.
 
Potable Water Supply: Water that is safe for human consumption and is aesthetically pleasing.
 
Non-potable Source: Any liquid, gas, or other substance that is not fit for human consumption and can adversely affect the quality of water.

The most common cross-connection is a typical garden hose, which is left submerged in a volume of water or other fluid (i.e. Garden hose left in a bucket of soapy water). Other cross-connections include kitchen sprayers in a sink filled with dirty water, irrigation systems, boilers, and other appliances supplied by water.

Backflow is the undesired reversal of flow of water and/or other substance into the potable water supply, via a cross-connection, due to a change in pressure caused by either Backpressure or Backsiphonage.
 
The potable water supply can be protected from backflow by eliminating all cross-connections and/or through the use of the proper backflow prevention assembly.
 
Backpressure: A type of backflow condition that exists when the pressure in the downstream side of the customers piping system becomes elevated greater than the supply pressure at the potable water system.
 
Backsiphonage: A type of backflow condition that exists when the pressure in the potable water supply system becomes lower than atmospheric pressure (14.7 psi).

A backflow prevention assembly or backflow preventer is a mechanical valve arrangement, which is designed to prevent the “backflow” or reversal of flow of water, once it has passed through the valve. There are different types of backflow preventers for different uses, and different degrees of hazard.