A butterfly valve is a type of flow control device used in various piping systems. It consists of a circular disc that rotates around a central axis to open or close the flow of fluid. When the disc is parallel to the flow, the valve is open, allowing fluid to pass through. When the disc is turned perpendicular to the flow, the valve is closed, stopping the fluid. These valves are commonly used in industries such as water treatment, oil and gas, and HVAC systems due to their simple design and quick operation. They are efficient, lightweight, and can handle large volumes of fluid with minimal pressure drop.