in simple terms,
the opposite of mainstream
underground rapper Canibus got some beef wit Eminem, who is mainstream.
by Marshall Mirzada December 4, 2003
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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
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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 26, 2003
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The Space of matter beneath the ground. Mostly thought to be considered only the earth's crust layer.
When someone dies, they are sometimes buried 6 feet underground.
by Michael Gatto August 29, 2008
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a zit that is deep, with no white head, painful, and bulbous, it takes a long time to head up and is generally super annoying.
dude, i got a huge undergrounder on my forehead!
by prot1a5 September 21, 2005
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Simply put, it’s an elite form of music or lifestyle not typical of the mainstream
Wow, Kyle wishes he was underground but just isn’t
by Undergroundguy420 February 22, 2020
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those fucking annoying pimples that are like a solid mound protruding from your head, hurt like hell, yet don't have a head so you can't deal with them
you: fml this new pimple is giving me a headache
friend: ouch, an underground?
by Catriine September 9, 2010
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