Definition of: underground

(un′dər·ground′) adjective
1. Situated, done, or operating beneath the surface of the ground.
2. Hence, done in secret; clandestine.
—noun
1. That which is beneath the surface of the ground, as a passage or space.
2. A railway operated in a system of tunnels beneath the ground.
3. A group secretly organized to resist or oppose those in control of a government or country.
4. An avant-garde movement in art, cinema, journalism, etc., generally considered to be in opposition to conventional culture or society and whose works are usually experimental, erotic, or radical in style, content, or purpose: used with the.
—adverb (un′dər·ground′)
1. Beneath the surface of the ground: to work underground.
2. Secretly.