Mainstream Media
a collection of mainstream sources of information.
like commercial. not underground. pop if u will
mainstream subjects
well, being "in the mainstream" usually means normal or ordinary, so, being out of the mainstream would mean unique or different. Depending on where you heard the word, there could be a negative connotation attached. For instance, some people use "not mainstream" to mean students that because of behavioral or learning difficulties can't be in a "normal" classroom with the other children. Other people just use it to mean separate, or different, sometimes in a good way.
If you Mean Music as in the Music industry/mainstream World... It's a Grammy.
mainstream is exocentric
It depends on what you mean by selling out. If you mean not signing to a major record label, then yes. If you mean not going mainstream, yes, but if a lot of people like what you are doing, you can automatically go mainstream without changing. If you are signed to a major record label, the label can force you to change your style.
the mainstream of American culture.
Mainstream Publishing was created in 1978.
Mainstream EP was created in 1998.
Mainstream - band - ended in 1999.