n. 1 a device for controlling the passage of fluid through a pipe etc., esp. an automatic device allowing movement in one direction only. 2 Anat. & Zool. a membranous part of an organ etc. allowing a flow of blood etc. in one direction only. 3 Brit. = thermionic valve. 4 a device to vary the effective length of the tube in a brass musical instrument. 5 each of the two shells of an oyster, mussel, etc. 6 Bot. each of the segments into which a capsule or dry fruit dehisces. 7 archaic a leaf of a folding door. valvate adj. valved adj. (also in comb.). valveless adj. valvule n.
[ ME f. L valva leaf of a folding door ]