WordNet sense 1 (a hollow cylindrical shape):

pipe

WordNet sense 2 ((anatomy) any hollow cylindrical body structure):

tube-shaped structure

WordNet sense 3 (electronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope):

electron tube, thermionic tube, thermionic vacuum tube, thermionic valve, vacuum tube

WordNet sense 4 (conduit consisting of a long hollow object (usually cylindrical) used to hold and conduct objects or liquids or gases):

tubing

WordNet sense 5 (an electric railway operating below the surface of the ground (usually in a city)):

metro, subway, underground, subway system