Definition of underground adjective from the Oxford Advanced Learner's Dictionary



    BrE BrE//ˌʌndəˈɡraʊnd//
    ; NAmE NAmE//ˌʌndərˈɡraʊnd//
    [only before noun] Committing crime, Terrorism
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  1. 1  under the surface of the ground underground passages/caves/streams underground cables compare overground
  2. 2operating secretly and often illegally, especially against a government an underground resistance movement The changes will affect all workers, whether underground or legal. The party was originally formed as an underground organization in 1987. See related entries: Committing crime, Terrorism
See the Oxford Advanced American Dictionary entry: underground