Definitions
1.nounAn underground railway, especially for mass transit of people in urban areas.
2.nounA train that runs on such an underground railway.
3.nounA rapid transit system, regardless of the elevation of its right of way.
4.nounAn underground walkway, tunnel for pedestrians (called pedestrian underpass in US).
5.nounAn underground route for pipes, sewers, etc.
6.nounRegions beneath the surface of the earth, both natural (eg. caves) and man-made (eg. mines).
7.noun(with definite article) A movement or organisation of people who resist political convention."the French underground during World War II"
8.noun(with definite article) A movement or organisation of people who resist artistic convention.
9.verbTo route electricity distribution cables underground
10.adjectiveBelow the ground; below the surface of the Earth."There is an underground tunnel that takes you across the river."
11.adjectiveHidden, furtive, secretive."These criminals operate through an underground network."
12.adjectiveOutside the mainstream, especially unofficial and hidden from the authorities."underground music"
13.adverbBelow the ground."The tunnel goes underground at this point."
14.adverbSecretly.