underground

UK: 'Underground': /ˈʌndəˌgraʊnd/; 'underground': noun, adverb: /ˌʌndəˈgraʊnd/, noun, adjective: /ˈʌndəˌgraʊnd/

US: /adv. ˈʌndɚˈgraʊnd; adj., n. -ˌgraʊnd/

US: (adv., un′dər ground′; adj., n., v. un′dər ground′)

WordReference Random House Learner's Dictionary of American English © 2023

un•der•ground /adv. ˈʌndɚˈgraʊnd; adj., n. -ˌgraʊnd/   adv. 

1. beneath the surface of the ground.
2. in hiding or secrecy;
   not openly:
   Their party went underground to fight the system.

adj. 

3. existing, situated, or operating beneath the surface of the ground.
4. hidden or secret;
   not open:
   underground political activities.
5. published or produced by political or social radicals:
   an underground newspaper.
6. experimental;
   avant-garde:
   an underground movie.

n. [countable]

7. the place or region beneath the surface of the ground.
8. Government a secret organization fighting the established government or occupation forces:
   the French underground of World War II.
9. Transport, British Terms [Brit.] a subway system.

WordReference Random House Unabridged Dictionary of American English © 2023

un•der•ground  (adv., un′dər ground′;adj., n., v. un′dər ground′), adv. 

1. beneath the surface of the ground:
   traveling underground by subway.
2. in concealment or secrecy;
   not openly:
   subversion carried on underground.

adj. 

3. existing, situated, operating, or taking place beneath the surface of the ground.
4. used, or for use, underground.
5. hidden or secret;
   not open:
   underground political activities.
6. published or produced by political or social radicals or nonconformists:
   an underground newspaper.
7. avant-garde;
   experimental:
   an underground movie.
8. critical of or attacking the established society or system:
   underground opinion.
9. of or for nonconformists;
   unusual:
   an underground vegetarian restaurant.

n. 

10. the place or region beneath the surface of the ground.
11. an underground space or passage.
12. Government a secret organization fighting the established government or occupation forces:
    He fought in the French underground during the Nazi occupation of France.
13. Government (often cap.) a movement or group existing outside the establishment and usually reflecting unorthodox, avant-garde, or radical views.
14. Transport, British Terms [Chiefly Brit.] a subway system.

v.t. 

15. to place beneath the surface of the ground:
    to underground utility lines.

• under- + ground¹ 1565–75

Collins Concise English Dictionary © HarperCollins Publishers::

underground adj /ˈʌndəˌɡraʊnd/

1. occurring, situated, or used below ground level: an underground tunnel, an underground explosion
2. secret; hidden: underground activities

adv /ˌʌndəˈɡraʊnd/

1. going below ground level: the tunnel led underground
2. into hiding or secrecy: the group was driven underground

n /ˈʌndəˌɡraʊnd/

1. a space or region below ground level
2. a movement dedicated to overthrowing a government or occupation forces, as in the European countries occupied by the German army in World War II
3. (as modifier): an underground group
4. the underground ⇒ an electric passenger railway operated in underground tunnels
   US and Canadian equivalent: subway
5. (usually preceded by the) any avant-garde, experimental, or subversive movement in popular art, films, music, etc
6. (as modifier): the underground press, underground music

WordReference Random House Learner's Dictionary of American English © 2023

sub•way /ˈsʌbˌweɪ/   n. 

1. Transport, British Terms an underground electric railroad:
   [countable] no delays this morning on the subways.
   [uncountable; by + ~]
   to travel by subway.
2. British Terms [countable] [Chiefly Brit.] a short tunnel or underground passageway;
   underpass.

WordReference Random House Unabridged Dictionary of American English © 2023

sub•way  (sub′wā′), n. 

1. Transport, British Terms Also called,[esp. Brit.,] tube, underground. an underground electric railroad, usually in a large city.
2. British Terms [Chiefly Brit.] a short tunnel or underground passageway for pedestrians, automobiles, etc.;
   underpass.

v.i. 

3. Transport to be transported by a subway:
   We subwayed uptown.

• sub- + way¹ 1820–30