Definition of “underground” - English Dictionary

“underground” in British English

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undergroundadjective, adverb

uk /ˌʌn.dəˈɡraʊnd/ us /ˌʌn.dɚˈɡraʊnd/

underground adjective, adverb (BELOW EARTH)

B2 below the surface of the earth; below ground:

an underground cave/passage/cable
Moles live underground.

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underground adjective, adverb (SECRET)

An underground activity is secret and usually illegal:

an underground newspaper/movement
The Communist Party was forced (to go) underground, and its leaders went into hiding.
the underground railroad

a secret system, used in the 19th century, by which slaves (= people who had been sold and forced to work) in the southern US were helped to escape to places where there was no slavery

undergroundnoun

uk /ˈʌn.də.ɡraʊnd/ us /ˈʌn.dɚ.ɡraʊnd/

underground noun (TRANSPORT)

the underground A2 [ S ] also the tube UK

a railway system in which electric trains travel through tunnels below ground:

the London Underground
They went on the underground.

(Definition of “underground” from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)

“underground” in American English

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undergroundadjective, adverb [ not gradable ]

us /ˌʌn·dərˈɡrɑʊnd/

below the surface of the earth; below ground:

an underground garage

Something that is done underground is secret or hidden, usually because it is not traditional or is shocking or illegal:

an underground newspaper
Officials believe the sighting of the suspect may have forced him to go underground (= to become secret).

undergroundnoun [ U ]

us /ˈʌn·dərˌɡrɑʊnd/

The underground is an organization that secretly works against those in power:

He was a member of the French underground in World War II.

(Definition of “underground” from the Cambridge Academic Content Dictionary © Cambridge University Press)