1. An instrument in which electrons move through a vacuum or gaseous medium within a gas-tight chamber; used to generate, to amplify, and to correct electric oscillations and AC currents.
2. An electronic apparatus that consists, typically, of a sealed glass bulb containing two or more electrodes.

It is used to generate, to amplify, and to rectify electric oscillations and alternating currents.

3. An electron device in which electricity is conducted by electrons moving through a vacuum or gaseous medium within an hermetically sealed envelope.

A tube can perform rectification, amplification, modulation, demodulation, oscillation, limiting, and a variety of other activities.

Examples include cathode-ray tubes, gas tubes, phototubes, and vacuum tubes.