butterfly valve

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. A kind of double clack-valve used in pumps. It consists essentially of two semicircular clappers, clacks, or wings hinged to a cross-rib cast in the pump-bucket, and is named from its resemblance to the wings of a butterfly when open, as represented in section in the annexed cut. It is employed in the lift-buckets of large water-pumps, and for the air-pump buckets of condensing steam-engines. Also called butterfly-cock. See clack-valve.

GNU Webster's 1913

1. n. (Mech.) a kind of double clack valve, consisting of two semicircular clappers or wings hinged to a cross rib in the pump bucket. When open it somewhat resembles a butterfly in shape.

WordNet 3.0

1. n. a valve in a carburetor that consists of a disc that turns and acts as a throttle