A butterfly valve is a type of flow control device used in pipelines to regulate the flow of liquids or gases. It consists of a circular disc or plate that rotates around a central axis, allowing it to open or close the flow path. When the disc is parallel to the flow, the valve is open, and when it is perpendicular, the valve is closed. These valves are popular due to their simple design, lightweight construction, and quick operation. They are commonly used in various industries, including water treatment, oil and gas, and HVAC systems, making them versatile for many applications.