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

      

    underground

     adjective
    adjective
    BrE BrE//ˌʌndəˈɡraʊnd//
     
    ; NAmE NAmE//ˌʌndərˈɡraʊnd//
     
    [only before noun] Committing crime, Terrorism
     
    jump to other results
  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