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1.
Un•der•ground (un’der ground’), n., adj.

1) A genre in music and other forms of media intended for an elite audience, that is often characterized by its high levels of originality and experimentation, and does not conform to typical standards, trends, or hypes as set by the popular mainstream media.
The mainstream media has a tendency to steal new ideas from the underground.
by Essix April 16, 2005
 
2.
Art, opinion or organization that exists outside of mainstream society or culture. Also known as "independent" or sometimes "counter-culture".
Giff preferred the underground music scene in her area to the bland stuff that played on most radio stations.
by Gyro June 09, 2002
 
3.
underground is like deeper beats, lyrics and soul to a song - commercial is like what get's played on the radio and is marketed to the masses - Britney's mainstream, Eminem is mainstream - even though anyone can make underground music they don't coz it doesn't sell like commercial stuff - underground IS deeper, more hardcore, more controversial, or whatever...
sup the chenist - dust c.d is underground.. you'll know the difference if you know your music...

NB: It isn't underground because it hasn't hit mainstream yet - it's whole design is different to mass appeal.
by TakticOne February 24, 2004
 
4.
Does NOT have to be unknown, it can also be a rapper that was underground yet kept the style and became extremely popular and well-selling commercial material
Pharoahe Monch and Wu-tang clan have music that is not bombarded with commercial material yet sell heaps
by Big Mac December 25, 2003
 
5.
in simple terms,
the opposite of mainstream
underground rapper Canibus got some beef wit Eminem, who is mainstream.
by Marshall Mirzada December 04, 2003
 
6.
Underground is about passion. of course the word is missused and abused . a true underground artist will create music/art from the heart as apposed to something tailored to a commercial market.an underground event organiser will still put on the event for the love of it even if its going to lose money... underground artists can have a long career out of the often distructive gaze of mainsteam media and earn money which is closer to someone with a good job than the millions of there commercial counterparts. where as a commercial artist will probably have a short shelf life
"I have been buying his records for 15 years, a true underground artist"

"whatever happened to him , last year he was on every tv show , radio station and all the teenage girls had his tee shirt"
by from the heart June 12, 2007
 
7.
The London railway system. Also called the Metro or Tube.
We took the Underground from King's Cross to Covent Gardens.
by Zen84 December 01, 2007