Noun

• S: (n) underground, resistance (a secret group organized to overthrow a government or occupation force)
• direct hyponym / full hyponym
• direct hypernym / inherited hypernym / sister term
• S: (n) revolutionary group (a political unit organized to promote revolution)
• S: (n) political unit, political entity (a unit with political responsibilities)
• S: (n) unit, social unit (an organization regarded as part of a larger social group) "the coach said the offensive unit did a good job"; "after the battle the soldier had trouble rejoining his unit"
• S: (n) organization, organisation (a group of people who work together)
• S: (n) social group (people sharing some social relation)
• S: (n) group, grouping (any number of entities (members) considered as a unit)
• S: (n) abstraction, abstract entity (a general concept formed by extracting common features from specific examples)
• S: (n) entity (that which is perceived or known or inferred to have its own distinct existence (living or nonliving))
• derivationally related form
• S: (n) metro, tube, underground, subway system, subway (an electric railway operating below the surface of the ground (usually in a city)) "in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'"

Adjective

• S: (adj) belowground, underground (under the level of the ground) "belowground storage areas"; "underground caverns"
• S: (adj) clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover, underground (conducted with or marked by hidden aims or methods) "clandestine intelligence operations"; "cloak-and-dagger activities behind enemy lines"; "hole-and-corner intrigue"; "secret missions"; "a secret agent"; "secret sales of arms"; "surreptitious mobilization of troops"; "an undercover investigation"; "underground resistance"
• similar to
• derivationally related form
• antonym

Adverb

• S: (adv) underground (in or into hiding or secret operation) "the organization was driven underground"
• S: (adv) underground (beneath the surface of the earth) "water flowing underground"