Top definition
The opposite of 'mainstream'. Applies to music (modern and older), books, hobbies, movies, etc.

The 'Is It Mainstream Music?' Test: Does this song/band play on more than three radio stations? Yes- Most likely main. No- Most likely sub.
Miley Cyrus and Jay-Z? Mainstream. Freezepop? Substream.
by Unexist May 16, 2010
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