butterfly valve

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A disk turning on a diametrical axis inside a pipe, used as a throttle valve or damper.
• noun A valve composed of two semicircular plates hinged on a common spindle, used to permit flow in one direction only.

from The Century Dictionary.

• noun A kind of double clack-valve used in pumps.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a valve in a carburetor that consists of a disc that turns and acts as a throttle