1. Also called, esp. Brit., vac'uum valve". an electron tube from which almost all air or gas has been evacuated: formerly used extensively in radio and electronics.
2. a sealed glass tube with electrodes and a partial vacuum or a highly rarefied gas, used to observe the effects of a discharge of electricity passed through it.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.