Final answer:
A backflow preventer is a device that stops water from flowing back when a hose is connected to a faucet.
Step-by-step explanation:
The device that prevents backflow when a hose is connected to a faucet is called a backflow preventer.
When there is a change in pressure, a backflow preventer valve lets water flow in one direction but stops it from flowing back. Water flows only in the desired direction, preventing contamination of the water supply.
The check valve is one kind of backflow preventer that is frequently used in hoses. When the pressure in the hose exceeds the pressure in the faucet, this valve closes, stopping water from returning to the faucet. Otherwise, water can flow freely from the faucet to the hose.