Valve  n. 1 device controlling flow through a pipe etc., esp. Allowing movement in one direction only. 2 structure in an organ etc. Allowing a flow of blood etc. In one direction only. 3 = *thermionic valve. 4 device to vary the effective length of the tube in a trumpet etc. 5 half-shell of an oyster, mussel, etc.  valvular adj. [latin valva leaf of a folding door]