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

Fleming staked much of his campaign on opposition to carbon capture and sequestration, the process for injecting carbon dioxide waste underground to reduce industrial pollution. — ABC News, 27 June 2026

As Aang sets out to rescue his majestic sky bison Appa and his Team Avatar friends, he’s joined by a surprise ally as Zuko’s uncle Iroh learns his nephew is trapped underground too. — Skyler Trepel, PEOPLE, 26 June 2026

Noun

When P-waves start radiating from the earthquake underground, Android phones sense the vibrations, start collecting data and send it back to Google servers for processing. — Amy Graff, New York Times, 27 June 2026

Invisible hands strap her legs onto the stirrups, an iron fist plunging her through a tunneling, gravity‑bending, black underground. — Literary Hub, 25 June 2026

Adjective

Inside, plans call for wet and dry research labs, teaching labs, collaboration areas, offices, instructional spaces and a 130‑space underground parking garage. — Gabby Sartori, USA Today, 30 June 2026

Some experts — and Kennedy — argue that offering peptides through licensed compounding pharmacies would shrink this underground market and direct people toward safer products. — Aria Bendix, NBC news, 30 June 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