Definitions for vacuum tube
Random House Webster's College Dictionary
an electron tube from which almost all air or gas has been evacuated.
tube, vacuum tube, thermionic vacuum tube, thermionic tube, electron tube, thermionic valve(noun)
electronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope
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.