1. a. A hollow cylinder, especially one that conveys a fluid or functions as a passage. b. An organic structure having the shape or function of a tube; a duct: a bronchial tube.
2. A small, flexible cylindrical container sealed at one end and having a screw cap at the other, for pigments, toothpaste, or other pastelike substances.
3. Music. The cylindrical part of a wind instrument.
4. Electronics. a. An electron tube. b. A vacuum tube.
5. Botany. The lower, cylindrical part of a gamopetalous corolla or a gamosepalous calyx.
6. Chiefly British. A subway; an underground.
7. A tunnel.
8. An inner tube.
9. Slang. a. Television: What's on the tube? b. A television set.