Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • adj. Situated, occurring, or operating below the surface of the earth: underground caverns; underground missile sites.
  • adj. Hidden or concealed; clandestine: underground resistance to the tyrant.
  • adj. Of or relating to an organization involved in secret or illegal activity: underground trade in weapons.
  • adj. Of or relating to an avant-garde movement or its films, publications, and art, usually privately produced and of special appeal and often concerned with social or artistic experiment.
  • n. A clandestine, often nationalist, organization fostering or planning hostile activities against, or the overthrow of, a government in power, such as an occupying military government: "an underground of dissident intellectuals” ( Kenneth L. Woodward).
  • n. Chiefly British A subway system.
  • n. An avant-garde movement or publication.
  • adv. Below the surface of the earth.
  • adv. In secret; stealthily.
  • transitive v. To situate under the ground: workers undergrounding telephone lines.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • adj. Below the ground; below the surface of the Earth.
  • adj. Hidden, furtive, secretive.
  • adj. Of music, art, etc, outside the mainstream.
  • adv. Below the ground.
  • adv. Secretly.
  • n. An underground railway.
  • n. A movement or organisation of people who resist political convention.
  • n. A movement or organisation of people who resist artistic convention.
  • v. To route electricity distribution cables underground

from the GNU version of the Collaborative International Dictionary of English

  • adj. Being below the surface of the ground.
  • adj. Done or occurring out of sight; secret.
  • adv. Beneath the surface of the earth.
  • n. The place or space beneath the surface of the ground; subterranean space.
  • n. a subway or subway system, especially in the United Kingdom.
  • n. a secret organization opposed to the prevailing government.
  • n. a group or movement holding unorthodox views in an environment where conventional ideas dominate, as in artistic circles.
  • prep. below the surface of the ground.

from The Century Dictionary and Cyclopedia

  • Beneath the surface of the earth: as, to sink underground.
  • Being below the surface of the ground: as, an underground story or apartment.
  • n. That which is beneath the surface of the ground.
  • To place or lay underground, as an electric wire.
  • n. An underground passage.
  • n. A contraction of underground railway.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • adj. conducted with or marked by hidden aims or methods
  • adv. in or into hiding or secret operation
  • adv. beneath the surface of the earth
  • adj. under the level of the ground
  • n. an electric railway operating below the surface of the ground (usually in a city)
  • n. a secret group organized to overthrow a government or occupation force

Etymologies

under +‎ ground. (Wiktionary)

Examples

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