What is a butterfly valve?
It’s an apparatus with an actuated disk-shaped componet that opens and closes to regulate the the flow of a fluid.
For example, a carburetor has a butterfly valve that opens to allows an air/fuel mixture into the cylinders and closes (not entirely, as air still needs to get in) when idling.
usually made of metal and circular, mounted across its diameter on a shaft so that the plate turns through 90 degrees when the shaft is twisted. The shaft is mounted on its extremities in a round tube allowing the plate (butterfly) to open or close the tube.