A butterfly valve is a type of quarter-turn valve used to regulate the flow of fluids within pipelines. It consists of a circular disc or plate mounted on a shaft in the center of the pipe. When the valve is open, the disc is turned parallel to the flow direction, allowing fluid to pass through with minimal obstruction. Conversely, when the valve is closed, the disc is rotated perpendicular to the flow direction, creating a tight seal that prevents the passage of fluid.