Oxford Advanced American Dictionary

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

underground

noun
 
underground pronunciation English ˈʌndəɡraʊnd underground pronunciation American ˈʌndərɡraʊnd
1 (often the Underground) (British English) (North American English subway) [singular] an underground railway/railroad system in a cityunderground stationsthe London UndergroundI always travel by underground.2 the underground [singular + singular or plural verb] a secret political organization, usually working against the government of a countryUsage notesUsage note: underground / subway / metro / tubeA city’s underground railway/railroad system is usually called the underground (often the Underground) in British English and the subway in North American English. Speakers of British English also use subway for systems in American cities and metro for systems in other European countries. The Metro is the name for the systems in Paris and Washington, D.C. London’s system is often called the Tube.