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 or blocking the passage of the fluid. When the disc is parallel to the flow, the valve is open, and when it is perpendicular, the valve is closed. These valves are known for their compact design and quick operation, making them suitable for various applications, including water supply, HVAC systems, and industrial processes. They are often preferred for their efficiency and ease of maintenance compared to other valve types.