Cross-Connection

A cross-connection is any physical connection or arrangements between two otherwise separate piping systems, one of which contains potable water and the other water of unknown or questionable safety, whereby water may flow from one system to the other, the direction of flow depending on the pressure differential between the two systems. (See Backflow and Back-Siphonage). -- American Standard National Plumbing Code (ASA A40.8-1955)