1. A long hollow object (usually cylindrical) used to hold and conduct liquids or gases; SYN. tubing.
2. A system of electrodes arranged in an evacuated glass or metal envelope; SYN. vacuum tube, thermionic vacuum tube, thermionic tube, electron tube, thermionic valve.
3. (Anatomy) Any hollow cylindrical body structure; SYN. tube-shaped structure.

Or electron tube in electronics, a glass tube containing gas at low pressure, which is used to control the flow of electricity in a circuit. Three or more metal electrodes are inset into the tube. By varying the voltage on one of them, called the grid electrode, the current through the tube can be controlled, and the tube can act as an amplifier. Tubes have been replaced for most applications by transistors. However, they are still used in high-power transmitters and amplifiers, and in some hi-fi systems.

1. Tunnel
2. British; Subway

Inner tube.

To convey in a tube.