underground

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Situated, occurring, or operating below the surface of the earth: underground caverns; underground missile sites.
2. adj. Hidden or concealed; clandestine: underground resistance to the tyrant.
3. adj. Of or relating to an organization involved in secret or illegal activity: underground trade in weapons.
4. 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.
5. 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).
6. n. Chiefly British A subway system.
7. n. An avant-garde movement or publication.
8. adv. Below the surface of the earth.
9. adv. In secret; stealthily.
10. v. To situate under the ground: workers undergrounding telephone lines.

Century Dictionary and Cyclopedia

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

Wiktionary

1. adj. not comparable Below the ground; below the surface of the Earth.
2. adj. Hidden, furtive, secretive.
3. adj. Of music, art, etc, outside the mainstream.
4. adv. Below the ground.
5. adv. Secretly.
6. n. chiefly UK An underground railway.
7. n. with "the" A movement or organisation of people who resist political convention.
8. n. with "the" A movement or organisation of people who resist artistic convention.
9. v. To route electricity distribution cables underground

GNU Webster's 1913

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

WordNet 3.0

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