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.

Art, opinion or organization that exists outside of mainstream society or culture. Also known as "independent" or sometimes "counter-culture".

The London railway system. Also called the Metro or Tube.

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

in simple terms,
the opposite of mainstream

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...

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