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.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "vacuum tube" in a sentence
"Considering their atmosphere, it's unlikely they ever would invent it for themselves — imagine how thick a vacuum tube has to be, how strong a pump you need!"