Open English Wordnet
Nouns
(n)tube, vacuum tube, thermionic vacuum tube, thermionic tube, electron tube, thermionic valveelectronic device consisting of a system of electrodes arranged in an evacuated glass or metal envelope
(n)metro, tube, underground, subway system, subwayan electric railway operating below the surface of the ground (usually in a city)“in Paris the subway system is called the ‘metro’ and in London it is called the ‘tube’ or the ‘underground’”