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1.
It being related to music is the only correct part of the last definition. A level reached by few, overground is when an underground artist becomes so popular that they can no longer be called underground. I would say that any underground artist who sells 1 mill+ copies of any one of their albums can be considered overground.
Tech N9ne, Tecca Nina, and Aaron Yates are the only overground artists that come to mind. . . they are all the same person if you didn't catch that.
by The Grench February 21, 2008
 
2.
adj.
The opposite of underground.

More oftenly referred to as mainstream or commercial.

Used to describe music (usually rap/hip hop) that is played on the radio just to sell records, as opposed to being quality music.

n.
Above the earth

Often instead of saying 'overground music' it is simplified to just 'overground' or 'the overground'.
Kid: "Hey, Did you hear that new joint;insert crappy radio single on the radio yet?"
Kid2: "Naw, I don't listen to the overground"
by SpoR November 28, 2005