Definitions:

1. an electric railway operating below the surface of the ground (usually in a city) noun
   • in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'
2. beneath the surface of the earth adverb
   • water flowing underground
3. in or into hiding or secret operation adverb
   • the organization was driven underground

Synonyms:

• belowground
• hugger-mugger
• metro
• resistance
• subway
• subway system
• tube