a structure, esp in the heart or a vein, that closes temporarily to obstruct passage of material or permits movement of fluid in one direction only
any of numerous mechanical devices by which the flow of liquid, gas, or loose material in bulk may be controlled, usu to allow movement in one direction only
a device in a brass musical instrument for varying the tube length in order to change the fundamental tone by a definite interval
any of the separate joined pieces that make up the shell of an invertebrate animal; specif either of the two halves of the shell of a bivalve mollusc
any of the segments or pieces into which a ripe seed capsule or pod separates
chiefly Brit a vacuum- or gas-filled device for the regulation of electric current by the control of free electrons or ions