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Definition of underground noun from the Oxford Advanced American Dictionary

     

    underground

     noun
    noun
    NAmE//ˈʌndərɡraʊnd//
     
    [singular]
     
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  1. 1the underground a secret political organization, usually working against the government of a country
  2. 2 an underground railroad system, especially in London underground stations compare metro, subway
See the Oxford Advanced Learner's Dictionary entry: underground