vacuum tube

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An electron tube from which all or most of the gas has been removed, permitting electrons to move with low interaction with any remaining gas molecules.

from The Century Dictionary.

• noun A sealed glass tube employed to examine the effects of a discharge of electricity through air or other gas rarefied or exhausted to the required degree.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun electronics An electrical device containing a heated filament inside an evacuated container (generally a glass tube) and used to produce rectification (a diode vacuum tube), amplification or other functions.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun electronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope