the opposite of mainstream. usually not the 'trend' and usually not what everybody else is doing. original and individual. can usually be identified as something original and/or something that will probably never become popular, but awesome nevertheless.
mainstream : conformists :: non-mainstream : individuals.

SOMETIMES alternative/emo/indie music. other music, such as pop punk, can be mistaken as non-mainstream.
by ruth February 14, 2005
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Pretty much the opposite of Mainstream music. Rarely, if ever, gets on the radio or top 40, and it's not usually well known to the general public. Also known as "Underground Music".
Non-mainstream music includes bands/artists such as Dream Theater, Porcupine Tree, Dälek, etc.
by 1729 February 15, 2009
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