Final answer:
Backflow is the term used to describe the unwanted reverse flow of liquids in a piping system.
Step-by-step explanation:
The term used to describe the unwanted reverse flow of liquids in a piping system is backflow. Backflow occurs when there is a change in the pressure gradient in the system, causing the flow of liquid to reverse its normal direction. This can happen due to factors such as a decrease in pressure downstream or the presence of a backflow prevention device.