underground Meaning, Definition & Usage

1. noun a secret group organized to overthrow a government or occupation force
   resistance.
   
2. noun an electric railway operating below the surface of the ground (usually in a city)
   tube; subway system; subway; metro.
   
   • in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'
3. adjective satellite under the level of the ground
   belowground.
   
   • belowground storage areas
   • underground caverns
4. adjective satellite conducted with or marked by hidden aims or methods
   hugger-mugger; clandestine; secret; hole-and-corner; surreptitious; cloak-and-dagger; hush-hush; undercover.
   
   • 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
5. adverb in or into hiding or secret operation
   
   • the organization was driven underground
6. adverb beneath the surface of the earth
   
   • water flowing underground

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