1. (noun) underground, resistance
a secret group organized to overthrow a government or occupation force
2. (adj) 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'"
3. (adj) belowground, underground
under the level of the ground
"belowground storage areas"; "underground caverns"
4. (adverb) clandestine, cloak-and-dagger, hole-and-corner(a), 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"
5. (adverb) underground
in or into hiding or secret operation
"the organization was driven underground"
6. (adverb) underground
beneath the surface of the earth
"water flowing underground"
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Definition of 'underground' 
