underground

1 of 3

adverb

un·​der·​ground ˌən-dər-ˈgrau̇nd 

1

: beneath the surface of the earth

2

: in or into hiding or secret operation

underground

2 of 3

noun

ˈən-dər-ˌgrau̇nd 

1

: a subterranean space or channel

2

: an underground city railway system

3

a

: a movement or group organized in strict secrecy among citizens especially in an occupied country for maintaining communications, popular solidarity, and concerted resistive action pending liberation

b

: a clandestine conspiratorial organization set up for revolutionary or other disruptive purposes especially against a civil order

c

: an unofficial, unsanctioned, or illegal but informal movement or group

especially : a usually avant-garde group or movement that functions outside the establishment

underground

3 of 3

adjective

un·​der·​ground ˈən-dər-ˌgrau̇nd 

1

: being, growing, operating, or situated below the surface of the ground

2

: conducted by secret means

3

a

: existing outside the establishment

an underground literary reputation

b

: existing outside the purview of tax collectors or statisticians

the underground economy

4

a

: produced or published outside the establishment especially by the avant-garde

underground movies

underground newspapers

b

: of or relating to the avant-garde underground

an underground moviemaker

an underground theater

Synonyms of underground

Noun

• resistance

Adjective

• clandestine
• undercover
• covert
• sneak
• private

Examples of underground in a Sentence

Adverb

They had been living underground as fugitives.

Noun

I've ridden on the New York subway, the Paris Metro, and the London Underground.

joined the underground while still a teenager

Adjective

The drugs are supplied through an underground network.

She loves the city's underground music scene.

Recent Examples on the Web

Adverb

Life underground is no longer the Pleasantville-like, cheerful place viewers first encountered in Season 1. —Aramide Tinubu, Variety, 20 Feb. 2026

The foundation story starts underground. —Allison Palmer updated February 19, Kansas City Star, 19 Feb. 2026

Noun

While the concept of finding naturally occurring stores of hydrogen underground is relatively new, Johnson said the process of drilling and producing hydrogen wells is well established science and uses the same rigs used to drill for natural gas. —Cami Koons, Iowa Capital Dispatch, 17 Feb. 2026

Montreal’s underground is the RÉSO. —Vivian Song, CNN Money, 17 Feb. 2026

Adjective

The district has 56 poles and 7,500 linear feet of overhead conversion to 11,165 linear feet of underground trench spanning 227 properties, according to a council agenda report. —Luke Harold, San Diego Union-Tribune, 22 Feb. 2026

In Poland’s Owl Mountains, an underground Nazi complex lies underneath the fairytale Baroque architecture of Książ Castle. —Maureen O'Hare, CNN Money, 21 Feb. 2026

Word History

First Known Use

Adverb

1598, in the meaning defined at sense 1

Noun

1594, in the meaning defined at sense 1

Adjective

1601, in the meaning defined at sense 1

Phrases Containing underground

• go underground
• the underground

Rhymes for underground

• aboveground
• battleground
• belowground
• paperbound
• turnaround
• ultrasound
• wraparound
• abound
• aground
• around
• astound
• background

Kids Definition

underground

1 of 3 adverb

un·​der·​ground ˌən-dər-ˈgrau̇nd 

1

: below the surface of the ground

an underground passage

2

: in or into hiding or secret operation

the political party went underground

underground

2 of 3 noun

un·​der·​ground

ˈən-dər-ˌgrau̇nd

1

: a space under the surface of the ground

especially : subway sense 2

2

: a secret political group

especially : an organized body working in secret to overthrow a government or an occupying power

underground

3 of 3 adjective

un·​der·​ground ˈən-dər-ˌgrau̇nd 

1

: being, growing, operating, or located below the surface of the ground

an underground stream

2

: conducted secretly