back-flow prevention valve

back Preposition

— Toward the rear. He went back behind the stands. — To a place again. I didn't like it, so I sent it back. He went back to the same house. They came back again.

back Noun

— The back is the rear part of something; it is the part in the other direction from the front. I went to the back of the house. — The back is the rear part of the human body. He had a scar on his back.

back Verb

— If you back something, you support it. The Republicans backed the bill. — If you back up, you move backward. He put the car in gear and backed right into the garage door.

flow Verb

— If something flows, it moves in a smooth and regular way. The river flows gently to the sea.

flow Noun

— A flow is a smooth regular movement of a liquid, gas, etc. A major accident is blocking the flow of traffic on Yonge St., just north of Queen.

prevention Noun

— The act of preventing something or hindering it; when someone or something is stopped from getting close, getting inside, or doing something. We built a wall to help the prevention of an attack on the city.

valve Noun

— A valve is part of a tube that lets liquid or gas go through one way but stops it from going through the other way. There are four valves in the heart that let blood go in one direction but stop it from going in the other direction. — A valve is part of a tube that someone can open or close to let water or other things go through. Open this valve to let water go through the pipe.