electron tube

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

• noun A sealed enclosure, either highly evacuated or containing a controlled quantity of gas, in which electrons can be made sufficiently mobile to act as the principal carriers of current between at least one pair of electrodes.

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

• noun physics Any of several devices consisting of a sealed chamber, either evacuated or filled with gas at low pressure, in which an electric current flows between electrodes.

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