Meaning of UNDERGROUND in English

UNDERGROUND

[noun] [U] [esp. UK]The underground (UK also tube, US subway) is a railway system in which electric trains travel along passages below ground.the London Underground systemThey went on the/by Underground.The underground is also a group of people who secretly fight against the government.a member of the undergroundThe role the underground played was to harass the occupying forces with a campaign of bombings and assassinations.The underground is also those people in a society who are trying new and often shocking or illegal ways of living or forms of art.In Britain and the USA in the 1970s the underground was a powerful subversive force.She began her career working in underground films.

Cambridge English vocab.      Кембриджский английский словарь.