Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its water supply must be isolated from the toilet bowl. For this reason, building codes mandate a series of measures to prevent backflow.
Backflow occurs for one of two reasons, either back pressure or back siphonage.
Back pressure is the result of a higher pressure in the system than in its supply, i.e. the system pressure has been increased by some means. This usually occurs in unvented heating systems, where thermal expansion increases the pressure.
Back siphonage is the result of supply pressure being lowered below that of the system. This occurs when a supply is interrupted or drained down.
in landscaping, backflow is when the water that is supposed to drain out of the yard, flows back into the main construction, causing serious damage to the foundation of the house.