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English definition of “underground”

underground

adjective, adverb uk   /ˌʌn.dəˈɡraʊnd/ us    /-dɚ-/

underground adjective, adverb (BELOW EARTH)

B2 below the surface of the Earth; below ground: an underground cave/passage/cable Moles live underground.The Earth and outer space - general words

underground adjective, adverb (SECRET)

describes an activity that is secret and usually illegal: an underground newspaper/movement The Communist Party was forced (to go) underground, and its leaders went into hiding.Secrecy and privacyLegal and illegal

underground

noun uk   /ˈʌn.də.ɡraʊnd/ us    /-dɚ-/

underground noun (TRANSPORT)

the underground A2 [S] (also the tube) UK a railway system in which electric trains travel along passages below ground: the London Underground They went on the underground.Railways and railway lines

underground noun (PEOPLE)

[U] 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.Nomadic and alternative lifestyles the underground a group of people who secretly fight against the government: He was a member of the underground, harassing the invading army.Taking action against people in powerActivism and pressure groups
(Definition of underground from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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