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

Potatoes planted in the garden a few months ago are forming potatoes underground and the plants are starting to decline. —Tom MacCubbin, The Orlando Sentinel, 25 Apr. 2026

Dickson said the site readiness work is the most complicated part of the project because of the amount of major mechanics that lie underground. —Jessica Moore, CBS News, 24 Apr. 2026

Noun

Household water largely comes from underground, the same aquifer that connects to the springs. —Letters To The Editor, The Orlando Sentinel, 24 Apr. 2026

The beetles winter underground, with adults emerging in late spring. —Mary Marlowe Leverette, Southern Living, 22 Apr. 2026

Adjective

In Paranal, specialists live like moles in an underground residence designed to keep their presence almost undetectable. —ABC News, 25 Apr. 2026

Reznik said while the San Gabriel River system and adjacent spreading grounds, stretching from Irwindale to Long Beach, work exceptionally well, capturing 90% of the local runoff that percolates into underground basins for later use, that’s not true of some other waterways in the county. —Steve Scauzillo, Daily News, 24 Apr. 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