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Definition of underground:

  1. Being below the surface of the ground; as, an underground story or apartment.
  2. Beneath the surface of the earth.
  3. Done or occurring out of sight; secret.
  4. The place or space beneath the surface of the ground; subterranean space.


underpass, partizan, covered, hole-and-corner, surface, immunity, tube-shaped structure, subterranean, subway system, subsurface, irregular, impedance, resistivity, resistor, resistance, radical, cloak-and-dagger, thermionic tube, hypogeous, subterrestrial, under, surreptitious, pipe, clandestine, covert, vacuum tube, partisan, guerrilla, on the quiet, thermionic valve, electric resistance, ohmic resistance, hush-hush, experimental, secret, guerilla, metro, hypogean, subway, thermionic vacuum tube, opposition, electron tube, hypogeal, buried, avant-garde, belowground, unusual, undercover, hugger-mugger, tubing, tube, electrical resistance.

  • secret (part of speech: adjective)

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