Definition of "underground" []

• Situated, occurring, or operating below the surface of the earth: underground caverns; underground missile sites. (adjective)
• Hidden or concealed; clandestine: underground resistance to the tyrant. (adjective)
• Of or relating to an organization involved in secret or illegal activity: underground trade in weapons. (adjective)
• Of or relating to an avant-garde movement or its films, publications, and art, usually privately produced and of special appeal and often concerned with social or artistic experiment. (adjective)
• A clandestine, often nationalist, organization fostering or planning hostile activities against, or the overthrow of, a government in power, such as an occupying military government: "an underground of dissident intellectuals” ( Kenneth L. Woodward). (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The term underground was used as symbolical of the secret manner in which our friends had to work in order to help us."
• "His 1957 essay “Underground films: a bit of male truth” coined the term underground film."
• "And about the underground religion, my understanding is that, well, those people who were going what we call underground or having covered religious activities because of the fear of being some negative effects on them -- I don't quite understand why those people should do this kind of thing."